http://www.instantaction.com/Someone at work told me about this today. They have multi-player flash games, but they're not your daddy's flash games. These are games (including a really good FPS) that are about on par with regular games from ~2002 and it plays from right within your browser. I was playing "Fallen Empire: Legion" for a bit tonight and it's very reminiscent of Tribes. Speaking of Tribes, I was told by the guy at work that this company/site was made by the guys who developed Tribes!
It looks like it's just in beta now, but they plan to make it a pay service eventually. I'd definitely recommend checking it out (especially while it's still free).