There has been a lot of discussion about metaclasses on c.l.py recently.

Ian McMeans asked for examples of the sort of thing that can best be done using metaclasses, and Mark McEahern demonstrates the use of metaclasses to implement aspect oriented programming in Python.

If this stuff makes your brains leak out of your ears, see Vladimir Marangozov's introduction to metaclasses. If it doesn't make your brains leak out of your ears, well, you are a lot cleverer than I am.

I linked to Jeff Epler's FinalMethods metaclass some time ago.

Aspect Oriented programming links here.