Skype, Lackey and OCTGN are the three big ones.
Skype's the biggest one and there's a Facebook group that has like 100+ people on it. Not too many posts from people on it nowadays but if you ask around there will probably be someone who will play a game. All that requires a webcam and a way to rig in so it points down in a birds eye view; everybody's got their own way of doing it (I just string my webcam through a multi-joint lamp).
Lackey's really easy to use but the card pool hasn't been updated in a while and there's barely anyone on it. OCTGN had a dedicated playerbase and I think the card pool has been updated but I've never used it because I never tried to figure out how to actually use the client.