C5 is a library of generic collection classes for the upcoming version 2.0 of the .Net platform. C5 extends the standard .Net System.Collections.Generic namespace with tree data structures, heap based priority queues and hash indexed array lists and linked lists, making it more comprehensive than most collection class libraries on similar platforms. Unlike many other collection class libraries, C5 is designed with a strict policy of supporting "code to interface not implementation".

Now that definitely looks worth checking out!