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

[Benjamin Pierce]

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!