Code of Tanks: 3-D PvP Tank Battle Game for Programmers
For those interested in .NET programming, you might like Code of Tanks, an online PvP tank battle game, brand new as of this writing. It does require some fundamental .NET programming knowledge, but other than that it's pretty simple: you create or join a room with other players, get your AI ready and pick your tanks, and watch the battle live from anywhere in 3-D.
It's in Beta and anyone can try it out: codeoftanks.com
No tournaments or contests yet, but live battles can be had any time. As a new product you may not find many competitors online when you are, but as more people discover it there may be rooms running around the clock. Until then you can just get some friends to play at the same time and run matches.