League of Legends - at first i was like meh about it just seeing it on videos. But then a friend kinda forced me into playing it and i love it now. Definetively worth a try
Team Fortress 2 - Hats. Properly the best free online shooter i have played(i have like 500hours into it XD) it is free, but you have a limited bank space and can't trade.. however you can pay for premium (5€)
Lord of the Rings Online is one of the most enjoyable F2P MMO's I have played. Beautiful and nostalgic environments taken from the books, and you can take the exact same path as the Fellowship of the Ring. Too bad the music is such a lackluster. Muting the game and playing Blind Guardian music in the background does the trick though.