There are plenty of great f2p MMOs out there.
The F2P model usually means if you want to see all of the content you have to purchase things, but you can usually get quite high playing just without, although you will miss out on the whole experience.
-Lord of the Rings online is a great F2P MMO, and is pretty well thought of overall. The PvP side of things isn't so good, but can't have everything.
- City of Heroes is a great Superhero game thats F2P also, and has some pretty fun PvP, but can be a bit simplistic, but if you just want something fun, give it a go. AMazing customisation also.
- Age of Conan is pretty good, quite recent, and has OKish PvP, but again some rave about it, some hate it, but its free so
Now, there are also some trial games-
-Warhammer Age of Reckoning has pretty damn good regular PvP, the trial is from 1-10 only, but the PvP is regular, fun and there are lots of classes to try, with lots of World PvP and BGs for you to play.
- Rift Free Trial - Pretty much WoW but better GFX, and some say better gameplay, only levels 1-20, but you get to see an awful lot of the game itself.
Give some of those a try, but there are hundreds of Free MMOs out there, but a lot are of dubious quality to say the least