If you don't find the collection you are looking for here (or in System.Collections.Generic or System.Collections.Specialized), try looking at the PowerCollections effort on GotDotNet: http://www.gotdotnet.com/content/featuredsite/powercollections/default.aspx
Also, You should look at the C5 generic collections (http://www.itu.dk/research/c5/). The C5 collections include the following generic collections:
ArrayList<T>,
CircularQueue<T>,
GuardedCollection<T>,
GuardedDictionary<K,V>,
GuardedDirectedEnumerable<T>,
GuardedEnumerable<T>,
GuardedEnumerator<T>,
GuardedIndexedSorted<T>,
GuardedList<T>,
GuardedQueue<T>,
GuardedSequenced<T>,
GuardedSorted<T>,
GuardedSortedDictionary<K,V>,
HashBag<T>,
HashDictionary<K,V>,
HashedArrayList<T>,
HashedLinkedList<T>,
HashSet<T>,
IntervalHeap<T>,
LinkedList<T>,
SequencedBase<T>,
SortedArray<T>,
SortedDictionaryBase<K,V>,
TreeBag<T>,
TreeDictionary<K,V>,
TreeSet<T>,
UnsequencedCollectionEqualityComparer<T,W>,
WrappedArray<T>