The Power Collections library, original developed by Wintellect at Microsoft's request is freely available on codeplex. The library includes many generic list classes that I wish were available directly in .NET. Here is a sampling:

Bag<T> - Collection of unordered T objects; hashed; duplicates allowed

OrderedBag<T> - Collection of ordered T objects; duplicates allowed

Set<T> - Collection of unordered T objects; duplicates not allowed

OrderedSet<T> - Collection of ordered T objects; duplicates not allowed

Deque<T> - Double-ended queue

OrderedDictionary<TKey, TValue> - Dictionary with ordered keys; one value per key

MutliDictionary<TKey, TValue> - Dictioniary where key can have multiple values; keys are hashed; duplicates allowed; items unordered

OrderedMultiDictionary<TKey, TValue> - Dictionary where key can have multiple values; keys ordered; duplicates allowed