For addictive fun (and free) online gaming, its hard to beat World of Tanks.
You start off with a garage of light tanks and one medium tank, and using experience points gained in battles you can upgrade them and also move on to even better armored units over time. You can then get other light tanks, medium tanks, heavy tanks, tank destroyers, and assault guns.
The battles are a lot of fun with lots of players on some very interesting maps and the system seems to balance the sides well so my tier 1 tank isn't getting clobbered by tier 3 or 4.
It's a pretty cool game indeed, and has a way of taking up a lot of free time.
Having played for a few hours I'm just starting to learn the basics of the game and to get some second tier tanks (and a 2nd tier tank destroyer) and it is neat how every tank has some significant performance differences.
It's well worth looking into if you like armored first person shooters.