Which packages make good use of S4 objects?

Question

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.