Wintellects's PowerCollections & PowerThreading libraries
Andru's blog post on the Wintellect PowerThreading library reminded me about the other excellent PowerCollections library that contains many highly-optimized collection & algorithm implementations.
I hadnt looked at the PowerCollections library since back in the early days of the .NET 2.0 beta's, and it looks like it has all grown up since that time. The great thing about these libraries is that they are FREE and open source (via Shared Source license, not GPL), so they are extensible and continue to be improved.