Perry Bible Fellowship is without question the best webcomic on the net. Every strip is funny, I look for my favourite and then just wind up reading 20 or so of them.

I also quite like the (one-man) Clone Army. Really a collection of webcomics, one is sick and perverted, one is cruel, one is dark and moody, one is... girly? and they're all done is a nice manga style.

Like most of the ones already mentioned, as well.