Well, I think it’s pretty cool, anyway.
Lookup allows you to have a collection of key-value pairs, but what makes it special is that you can have duplicate keys, which your normal Dictionary or SortedList don’t allow. This allows you to do all sorts of grouping and what not which is very useful.
The only downside is that you can’t new one up, you have to do something like myList.ToLookup(blah, blah, blah) to get one.
Check it out.