I'm using fluxbox for a couple of years now, after being a long-time fvwm user and getting sick of it. Though, I don't do anything very fancy (transparency, slit) with it, and in fact have gone to considerable lengths to make it look boring, lol
For those who like playing with new wm's just for
fun, a couple that caught my attention while searching were:
ratpoison A different, tiled, paradigm (not for me).
evilwm Fairly minimal. This might be worth a look for someone wanting to write their own wm, since the code is so small.