The
RPGResourceMasters is a great place to play online RPGs. They specialise in non-D&D systems. Note that this is not a criticism of D&D in it's various incarnations ... just that as you point out yourself, there are lots of web resources for D&D already.
I'm sure some of the other guys (or even the board admin - Ictus) will be along with a more impressive add soo enough ...