Guild Wars 2, it has no monthly fee and is just as good if not better than anything else out there.
There are F2P (free to play) mmo's too, but they are generaly lacking when compaired to subscription games like WoW, Rift or EverQuest2.
For F2P Allods is the best I've played, probably Runes of Magic is second.
Guild Wars 2 is a good choice. No monthly fee, a fairly nice community (especially compared to other online games like Blizzard's), easy enough to learn. Also, unlike many MMOs, you can take a decently long break from the game and not come back to find out everyone has progressed ridiculously far ahead of you. I play it off and on and never feel like my breaks really hurt me aside from losing familiarity with how the game plays.
It's a pretty fun world to explore as well. I was always thrilled when I stumbled across puzzles out in the world. It's also a lot of fun when you find a few people and just run around doing events with them. It's a very cooperative friendly environment since there's no mob or loot stealing in the game.
The biggest downside to me is that it's no longer new so the players are spread out in the world a lot more. It's not uncommon to be playing in a zone and not see many players. It helps to find some other players and play together.
It's also a bit unfortunate that your default bank space is so small, but you can upgrade the size. They have a cash shop (buy gems which you use to buy stuff) but you don't have to spend any money. You can buy gems with real money as well as in game gold. I actually purchased bank upgrades by exchanging gold for gems.
Edit: Also, GW2 having a cash shop does not mean it's pay2win. Far from it actually.
Edited by Ringo#1617 on 1/19/2013 2:16 PM PST
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