Yesterday Microsoft made available Community Technology Preview (CTP) of Service Pack 3 for SQL Server 2008. This allows you to test the service pack in your test or QA environments before deploying the eventual final release, and also to provide feedback to Microsoft about potential issues with the service pack. The build number is 10.00.5416.
You can download the CTP here (most likely you'll want the big file for your platform, named SQLServer2008SP3-KB2546945-[platform]-ENU.exe):
If you are using Express, the link is here:
You can see the list of fixes in KB #2546945:
And you can read the Release Services blog for more information.
This includes fixes from SP2 Cumulative Updates 1, 2, 3 and 4, but the updates from CU5 (KB #2555408) are *not* included. If you are relying on specific fixes from that build, you should hold off on installing any version of SP3 until the first post-SP3 cumulative update is available.
Note: the SP3 CTP is for SQL Server 2008 only. It is *NOT* for SQL Server 2008 R2. If SELECT @@VERSION returns 10.50.xxxx, please do not waste time downloading this update.