Which R packages make good use of S4 classes? I'm looking for packages that use S4 appropriately (i.e. when the complexity of the underlying problem demands), are well written and well documented (so you can read the code and understand what's going on).
I'm interested because I'll be teaching S4 soon and I'd like to point students to good examples in practice so they can read the code to help them learn.
Thinking about this some more, maybe Matrix and/or lme4? Matrix does a lot of trickery with efficient representation of sparse matrices so this may be a worthwhile (though possibly heavy) example.
Else, given that all of BioConductor is done in S4, some of it is bound to be better than average :) I am sure Martin Morgan will pipe in with good examples.