I play a lot of Age of Empires II online. I used to play it when I was young, but the online interface sucked back then, so I mostly played LAN games. Now that steam has re-released the game with an improved interface it's the best online gaming experience I've had since the days of BF2.
I also play a lot of insurgency. For anybody who likes Counter Strike or realistic tactical shooters, insurgency is a must have. I got it for dirt cheap, like $6 on sale. The gameplay is a lot like Counter Strike, with short rounds. That said, it's much more realistic than CS - Its based on the Iraq war, and is pretty historically accurate compared to most shooters. You also die with fewer shots, and it's a lot harder to quick scope. The maps are way better IMO, as there is no OG CS crowd that has to be appeased (meaning nothing can be changed). The best part about it is that it's made by a small independent company, yet it has grown incredibly popBFular among mainstream gamers. The graphics are also really good while still running very smoothly. Probably the best optimized game I've played since COD4 (which had great graphics and ran well, but doesn't have the quality gameplay Insurgency has).