For those developing with the Whidbey beta, there is a PowerCollections library produced by Wintellect in association with Microsoft. This library is essentially an upgrade of the existing .NET Collections API and has the distinct possibility of eventually migrating to or forming the basis of some future version of the .NET Platform.
It includes a several new collections missing in Whidbey such as Bag (ordered and unordered), Sets(ordered and unordered), and Deques. There is also a highly efficient BigList class support ultrafast insertions, deletions, copies and concatenation. All of these classes are free to use.
The main developer of PowerCollections had read and linked my article What's Missing in .NET and Other Collection Libraries at one point prior to creating the BigList class. In the article, I discussed missing collection types, especially a highly scalable list class with logarithmic time insertion and deletion times. I suspect also that this post also prompted members of BCL (either directly or indirectly) to seek the creation of the PowerCollections project due to the project being initiated shortly after the posting of my article.