Remember the old tank game where you changed your tank gun's elevation and accounted for the wind? How about the Worms series which did the game thing? Gunbound takes it on line!
It's free to sign up and free to play. You choose your tank to get started in. Each has its benefits and weaknesses - better defense, better speed, etc. The more you play, the more points you earn. You can trade those in to buy outfits, goggles, weapons and other cool treats for your avatar. Each gives you special bonusses and lets you show off your talent to the other players.
Gunbound is based in Korea and is currently being beta-tested in English, which is why it's free. Be sure to get in and enjoy the game before they turn it into a for-pay server! Even then, it might be fun enough to pay some small amount ...
If you want some fun, check out the original, Korean-language Gunbound site!