SQL Server 2005 service pack 3 released today and you can download it from here:
One of the most interesting and useful bits which I am interested in (apart from the bug fixes rolled up from all the cumulative updates) is the new ability to manage to the 'TokenAndPermUserStore' cache with more control. Many people have come across these performance problems, which I won't recount in detail as they are well documented here and here.
However one of the great new features in SP3 is the ability to control the size and entry count of this cache yourself, much like the feature that was introduced in SQL Server 2008 as part of the sp_configure settings.
In SQL 2005 SP3 you have to use a trace flag and some registry entries as opposed to sp_configure, but the behavior is much the same, and should allow people to workaround this problem in a less aggressive way than just clearing out the whole cache. The exact details of how to do this are documented here:
How to customize the quota for the TokenAndPermUserStore cache store in SQL Server 2005 Service Pack 3 - http://support.microsoft.com/kb/959823/