I've played countless free to play games in my time, so I'll just list the top three.
League of Legends (I reference it in almost every post it seems.

) I play it a ton, and have put 20$ into the game.
Atlantica Online: I played the game a lot, but once you get into the meat of the game it slowly becomes more and more of a P2W game. I refuse to support games that pretend to be free, so I put 0$ into it.
America's Army Operations. Played it just after beta. loved the heck out of that game, right up until they changed it to America's Army operations 3.0 and completely ruined the fun of it. It was 100% free (because it's basically a giant advertisment for the US Army) but if they had put an option to pay for some aethetic items I would've put money down in a heartbeat.
Also, WoW isn't free to play, they're just pretending it is. All they did was turn their 14 day trial into an untimed trial. You can't get past level 20, and you're super limited in what you can gather in game (I think you're capped at 4 gold and super-hefty restrictions on chat)
Edit: Oh wow! Navy field. Totally forgot about that one. (really it should be #2 in my list) Loved playing as a carrier in that game. Another game I put 0$ in, but I still had a lot of fun with that one as far as I got in it.
Edited by LackofCertainty, 06 March 2012 - 07:04 PM.