I use to play an old game called Battlefield on Bonus.com which was shut down. But, I just recently discovered it's back as TankPit . It's even better than before and so are the players... Back in the day I thought I was pretty good, but after playing TankPit for a week I realize I know nothing :).
I can't believe such a simple game can be so intense and additing. Out of all the high graphic games out there this is honestly the only one I play.
I have huge respect for the people who have put the work into bringing this back, and for the serious players who make it awesome!