Recently Dave Ballantyne posted a list of free resources for SQL Server, entitled "The best things in life are free."  Now, I'm not trying to upstage Dave, but I know of several resources that aren't on his list.  It would be good to have a true community page for this kind of thing, but for now a blog post will have to do.  [I also just re-noticed Mladen Prajdic's post on free SQL Server tools, and his list is quite exhaustive.]  Please leave a comment if you spot any errors, omissions, broken links, etc.
 

Free Upgrade Advisors / Best Practices Analyzers

These tools from Microsoft can be instrumental in cleaning up your systems and finding issues *before* the day of the upgrade.  I'm including the SQL Server 2005 versions, but if you're just planning your upgrade to 2005 now, I'd suggest re-thinking toward 2008 or 2008 R2.  The last service pack for 2005 is due out later this year, and the next major version of SQL Server is probably about a year after that.  Note that there is no SQL Server 2008 Best Practices Analyzer, at least that I could find.

SQL Server 2005 Upgrade Advisor

SQL Server 2005 Best Practices Analyzer
 

SQL Server 2008 Upgrade Advisor – download x64 – download x86

SQL Server 2008 R2 Upgrade Advisor – download x64 – download x86

SQL Server 2008 R2 Best Practices Analyzer – download x64 – download x86

(For some advice about the R2 BPA install, see this post from August)

SQL Server 2012 CTP3 Upgrade Advisor – download x64 – download x86
(Note that if you're installing RC0 or later, there is a newer Upgrade Advisor with the media, they just haven't made the feature pack public yet, as of 2012-01-13.)

Free Documentation

Books Online is the official go-to documentation for SQL Server, but you don't have to have a SQL Server license to use it.  You can choose to download the application (or install it as part of setup) and keep it up to date, or use the web-based version.

SQL Server 2008 Books Online – or download

Free Training Kits

SQL Server 2012 RC0 Developer Training Kit
http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=27721

Free Feature Packs

Jason Howell has summed up the current set of feature packs (these are utilities provided by Microsoft to complement SQL Server tools and services):

http://blogs.msdn.com/b/jason_howell/archive/2010/10/25/list-of-sql-server-feature-packs.aspx

Free Management Studio Add-Ins

Several authors have developed tools that plug into Management Studio, though I wouldn't expect all of them to work flawlessly with SQL Server 2008 R2 – yet.

Mladen Prajdic's SSMS Tools Pack:
http://ssmstoolspack.com/

Jonathan Kehayias' Extended Events Manager:
http://extendedeventmanager.codeplex.com/

RedGate SQL Search
http://www.red-gate.com/products/SQL_Search/

Daniel Gould's Internals Viewer:
http://internalsviewer.codeplex.com/

DevArt SQL Complete Express:
http://www.devart.com/dbforge/sql/sqlcomplete/download.html

SQL Sentry Plan Explorer:
http://www.sqlsentry.net/plan-explorer/sql-server-query-view.asp

Free Community Resources

Looking for other SQL Server people to help you – for free?  The SQL Server community is, as a whole, one of the best free resource you're likely to find in any industry.  I don't think I'm going to try and list out all of the SQL Server blogs here… there are just too many.  But I can point out places where you can get free help online or – believe it or not – in person:

Twitter hashtag : #sqlhelp
http://twitter.com/#search?q=%23sqlhelp

Also see this great twitter primer from @BrentO:
http://www.brentozar.com/archive/2008/08/twitter-101/

And more specifically, this summary of the #sqlhelp tag:
http://www.brentozar.com/archive/2009/12/i-need-sqlhelp/

SQL Server forums on MSDN:
http://social.msdn.microsoft.com/Forums/en/category/sqlserver/

Stack overflow:
http://stackoverflow.com/

DBA StackExchange:
http://dba.stackexchange.com/

Ask SQLServerCentral:
http://ask.sqlservercentral.com/

ServerFault:
http://serverfault.com/

SQLServerPedia is like Wikipedia but for SQL Server:
http://sqlserverpedia.com/

Also look for a SQLSaturday coming to a city near you:
http://sqlsaturday.com/

Or a local user group.  See this editorial by Brad McGehee:
http://www.sqlservercentral.com/articles/Editorial/67191/

Finally, PASS is free to join. Watch for free online training events and presentations via virtual chapters:
http://sqlpass.org/

Free Blog Assistance

This one is kind of sitting in its own category, but I didn't know where else it would fit.  This web form allows you to take T-SQL code and format it nicely – color coding and all – for various blog formats.  I have found it invaluable over the past few years in adding code samples to my posts:

http://extras.sqlservercentral.com/prettifier/prettifier.aspx

Free Sample Databases

You can download a wide variety of sample databases (mostly in the AdventureWorks flavor):
http://msftdbprodsamples.codeplex.com/

If you want to load Pubs and/or Northwind on a 2005+ server, you can, I guess:
http://www.microsoft.com/downloads/en/details.aspx?FamilyID=06616212-0356-46a0-8da2-eebc53a68034

Free Maintenance / Performance Scripts

Adam Machanic's Who Is Active? v.10.0:
http://sqlblog.com/files/folders/release/entry29675.aspx

Michelle Ufford's Index Defrag Script, v4.0
http://sqlfool.com/2010/04/index-defrag-script-v4-0/

Ola's SQL Server 2005 and 2008 – Backup, Integrity Check and Index Optimization
http://ola.hallengren.com/

Brent Ozar's BLITZ! 60 minute SQL Server takeover script
http://www.brentozar.com/sql/blitz-minute-sql-server-takeovers/

Kimberly Tripp's sp_helpindex2:
https://www.sqlskills.com/blogs/kimberly/sp_helpindex-v20170228/

Tibor Karazsi's utility procedures:
sp_indexinfo: http://www.karaszi.com/SQLServer/util_sp_indexinfo.asp
sp_tableinfo: http://www.karaszi.com/SQLServer/util_sp_tableinfo.asp

Free Maintenance / Performance Tools

SQL Sentry Plan Explorer:
http://www.sqlsentry.net/plan-explorer/sql-server-query-view.asp

SQLCop:
http://sqlcop.lessthandot.com/

Microsoft's SQL Server Backup Simulator:
http://blogs.msdn.com/b/sqlserverfaq/archive/2010/10/27/sql-server-backup-simulator.aspx

Confio IgniteFree:
http://www.ignitefree.com/

PSSDIAG (for SQL Server 7.0 / 2000):
http://www.microsoft.com/downloads/en/details.aspx?familyid=5564386A-28C2-4483-8293-76FFF67B9EB3

SQLNexus (for SQLDiag / PSSDiag):
http://sqlnexus.codeplex.com/

SQLQueryStress:
http://www.datamanipulation.net/sqlquerystress/

SQL Load Generator:
http://sqlloadgenerator.codeplex.com/

SQLIOSim:
http://blogs.msdn.com/sqlserverstorageengine/archive/2006/10/06/SQLIOSim-available-for-download.aspx

Performance Analysis of Logs (PAL) Tool:
http://pal.codeplex.com/

scaleSQL ClearTrace:
http://www.scalesql.com/cleartrace/

Qure Workload Analyzer:
http://www.dbsophic.com/qure-workload-analyzer/

Joe Chang's SQL Exec Stats (docs):
http://www.qdpma.com/ExecStatsZip.html

AutoAudit:
http://autoaudit.codeplex.com/

SQL Server Cache Manager:
http://sqlservercache.codeplex.com/

SQL Server 2005 Performance Statistics Script:
http://blogs.msdn.com/b/psssql/archive/2007/02/21/sql-server-2005-performance-statistics-script.aspx

SQL Server 2005 Performance Dashboard Reports:
http://blogs.msdn.com/b/sqlrem/archive/2007/03/07/performance-dashboard-reports-now-available.aspx

SQL Server 2008 Performance Dashboard Reports:
http://blogs.technet.com/b/rob/archive/2009/02/18/performance-dashboard-reports-for-sql-server-2008.aspx

SQL Server Health and History Tool (SQLH2):
http://www.microsoft.com/downloads/en/details.aspx?FamilyID=EEDD10D6-75F7-4763-86DE-D2347B8B5F89
Performance Collector: http://www.microsoft.com/downloads/en/details.aspx?familyid=64983AF0-7902-427E-9B41-7C2E8FDCC140
Performance Reports: http://www.microsoft.com/downloads/en/details.aspx?familyid=A4B837C7-A1FA-4F25-840B-FEF15E917F18

SqlMonitoring Tool:
http://sqlmonitoring.codeplex.com/

Log Parser 2.2:
http://www.microsoft.com/downloads/en/details.aspx?FamilyID=890cd06b-abf8-4c25-91b2-f8d975cf8c07

DMVStats 1.01:
http://sqldmvstats.codeplex.com/

Free Configuration / Policy Tools

Scripted PowerShell Automated Deployment Engine (SPADE):
http://sqlspade.codeplex.com/

Quest Discovery Wizard beta:
http://www.quest.com/discovery-wizard-for-sql-server/

Enterprise Policy Management (EPM) Framework:
http://epmframework.codeplex.com/

Partition Management:
http://sqlpartitionmgmt.codeplex.com/

SQL Server Central Management System:
http://sqlcms.codeplex.com/

SQL Server Express Utility:
http://www.microsoft.com/downloads/en/details.aspx?FamilyID=fa87e828-173f-472e-a85c-27ed01cf6b02

Free SQL Server IDEs

Various free tools from Atlantis:
http://www.atlantis-interactive.co.uk/

Toad for SQL Server has a freeware version:
http://toadforsqlserver.com/index.jspa

Free SQL Server eBooks

I was really reluctant to list all of the eBooks I could find, because it is often difficult to asses the legality of many offers out there.  Here are a few that I know are legit, and I'll let you search for others on your own.

SQL Server Crib-Sheet Compendium (Amirthalingam Prasanna, Grant Fritchey, Phil Factor, Robert Sheldon, Robyn Page):
http://www.simple-talk.com/sql/learn-sql-server/free-ebook-sql-server-crib-sheet-compendium/

SQL Server DMV Starter Pack (Glenn Berry, Louis Davidson, Tim Ford):
http://downloads.red-gate.com/ebooks/SQLServer_DMV_StarterPack.zip

Defensive Database Programming (Alex Kuznetsov):
http://www.red-gate.com/specials/Ebooks/defensive_database_programming.htm

SQL Server Execution Plans (Grant Fritchey):
http://www.red-gate.com/specials/Grant.htm

Introducing SQL Server 2008 R2 (Microsoft Press):
http://download.microsoft.com/download/C/D/1/CD1648C1-46BC-4C79-B16F-7AF650C186B4/693883ebook.pdf

Brad's Sure Guide to SQL Server 2008 (Brad McGehee):
http://www.red-gate.com/specials/ToolbeltBradsure.htm

Paul Randal's DBA-myth-a-day series
http://www.sqlskills.com/BLOGS/PAUL/Myths-and-misconceptions-60-page-PDF-of-blog-posts/

Free SSIS Tools and Resources

SSIS Community Tasks and Components
http://ssisctc.codeplex.com/

Free PowerShell Tools and Resources

PowerGUI:
http://www.powergui.org/downloads.jspa

Free PowerShell IDE:
http://www.hanselman.com/blog/FreePowerShellIDE.aspx 

PowerShell Extensions (SQLPSX):
http://sqlpsx.codeplex.com/

TechNet : Scripting with PowerShell:
http://technet.microsoft.com/en-us/scriptcenter/powershell.aspx

PowerShellPack:
http://archive.msdn.microsoft.com/PowerShellPack
 

Free PowerShell eBooks

An Introduction to Microsoft PowerShell
http://download.microsoft.com/download/a/9/4/a94270c7-ed16-4c72-8280-658c66315719/Windows%20Powershell%20-%20EN.zip

Effective Windows PowerShell
http://keithhill.spaces.live.com/blog/cns!5A8D2641E0963A97!6930.entry?sa=58811724

PowerShell Workbook : server administration
http://blogs.technet.com/b/chitpro-de/archive/2008/02/28/free-windows-powershell-workbook-server-administration.aspx

Master-PowerShell:
http://powershell.com/cs/blogs/ebook/