World of Tanks straddles the definition of MMO. It's massive, and multiplayer, but instead of a persistent world, it consists of battlefields and scenarios you spawn into. And you play... a tank. Or, rather, many tanks, as you upgrade your skills, unlock new vehicles, and customize them with increasingly advanced technology and weaponry, from the earliest World War I clunkers to the modern war machines.
The graphics, sound effects, and "feel" of the tanks was good; they are sluggish and powerful. While the World of Tanks gameplay is aim-and-shoot, it requires more precision and less twitching, which is not to say I was very good at it.
World of Tanks is free, but vendor Wargaming.net runs a premium shop that allows purchase of superior weapons and gear. The community is, well, what you'd expect from a game highly focused on tournament-style competitive play. Don't expect any handholding.
Note: The Download button takes you to the vendor's site, where you can download the software.