For years, ever since I started posting build lists over at aspfaq.com (don't go there now; it's been taken over by malware slimeballs), I have been complaining that Microsoft makes it very hard for us to stay on top of these things, and suggested on numerous occasions that they publish a list of builds that have been released, even if only through support cases.
Well, it looks like they're finally listening. Here is a list of all of the builds that have been released for SQL Server 2005, since service pack 2:
Now what I would suggest next is that whenever they create a summary KB article like this, they place it somewhere prominent (e.g. on the service pack x download page). This way, people don't have to know to take it upon themselves to go search for post-SP builds that will fix any unexpected problems that the service pack itself introduces (I'll give you a hint: sounds like "tainted Jets fans").
So, people who have been painstakingly keeping track of builds manually (myself on this blog, and others like sqlserver-qa.net, it seems like we have much less work cut out for us in the future. The main problem is that it's impossible to keep updating our own blog entries or reference sites, especially during periods of high activity for the build team (currently we're not in one of those). Hopefully they will keep improving the usability of the site and figure out a way to make these lists of builds more prominent and easier to find… more importantly, easier to figure out that we need to look for them.