Going to ramble a bit, since I love PnP RPGs...
My game group primarily does all sessions online (since we're scattered all over the place and not in one general area). Hard to get everyone together at an appointed time to play, but we manage. Luckily, one of the players has just returned from an army stint in Afghanistan, so it looks like the group might start up again. (we havent played much since he's over there, w/ a connection made of suck).
Lessee.. we've used lots of different things online to play, although we tend to prefer the simple things, like MSN messenger. We loved using Google Wave, but that's gone. We've been debating about maybe using Google+ to run a session (involving webcams and such).
Out of our group of... i think its 6 people, on average, we can usually get 5 of us to play, and sometimes bring in a extra person or two. We tend to take turns as to who's the GM, and what systems we play, but I tend to be the primary GM for the group.
We've played... DnD4th Ed, Shadowrun, Maid RPG (yes, don't laugh, awesomely stupid game), to name a few. Also have run some homebrew rules here and there.
Great, now I have an urge to play soon.... I'll need to look through all my rule books and choose a game system to play. I have been annoying the group to play some Paranoia... but they seem to be deathly afraid of playing it, if I'm the GM for some reason.