One of the missing collection classes in .Net is one that's not as often used, but when you need it, boy, it's really missing, is the Set. You know - Set theory (”Group Theory”) and all, including Intesect, join and so on.
I found this really nice Set implementation library with Source over on CodeProject :
You might also want to read the discussion at the bottom of the article, some nice points there.
Another link related to this is that someone created a more performance oriented variant of the Set - LargeListDictionary, on CodeProject, based on this article. It might come in Handly some day too:
(via win-tech-off-topc mailing list)