[Haskell-cafe] Nice pedagogical examples of type classes?

My Advanced Programming course is quickly approaching the lectures on type classes, and I am interested in finding a little more (beyond what's in SOE) in the way of examples that illustrate nice uses (especially of more advanced aspects of the class system). I'd be most grateful for pointers to people's favorites. Examples should be reasonably small and depend only on Haskell 98 features. I haven't discussed monads yet, but I'm also interested in good monadic examples for later in the semester. Many thanks, - Benjamin P.S. Suggestions for interesting exercises along these lines are also appreciated!