Urban Dead is just about the only one I'd really recommend to anyone else looking a free game. Also, there's Die2Nite, which is sort of like Urban Dead's cooperative/town-building counterpart.
AvA in itself is always rife with hackers. And 12 years-olds. Perhaps if you really, really, really can't afford a game (which I doubt, everyone has some sort of stable income these days, and Team Fortress 2 is $10 at the gameshop), then it might be worth a play. Might.
Maplestory isn't one of my particular favorites, but it's violence-lite, so I recommend it for anyone who has free time or wishes to get into MMOs. The art style/gameplay of the game is hit or miss, though (it was miss for me).
Vindictus is a mixed bag. I absolutely hate it because Nexon doesn't seem to realize that everywhere else other than Korea has pretty variable internet (read: mostly unstable and not always high-speed government-mandated like Korea's is), and I think it's a stupid move to keep the system peer-to-peer based instead of moving it onto server-based instancing (which would save a lot of trouble of just trying to find a damn good party instead of dropping out repeatedly, getting frustrated, and ending up running the mission yourself, which sucks, too).
Anyways, for my recommendations:
Battlezone (1998 remake): I recommend this WITH HIGHEST HONORS. It's absolutely free, just google up Battlezone Enhanced (make sure it's by a guy named Spock. It contains the singleplayer, which is what the game is all about: first-person tactical command).
Dark Reign: Starcraft's lesser micro-cousin and more about massive armies waging warfare. Defense is killer in this one (after all, giant gun towers annihilate most things). The game's alright, but it's easy to see how Starcraft overshadowed it.
Netstorm: Floating islands! Magic! Building bridges to attack your enemy's island! It's a load of fun (and very thinking-intensive). Try it out!