I've tried a lot of free MMO's, but MegaTen really cought my atttention!
The graphics are better than any other free MMO out there. It's possible to set a custom resolution, so no matter what, it will fit your monitor 100% with smooth graphics.
The game is set in a near future where demons roam the Earth. As you begin the game, you take the role of a Demon Buster, and you have the abilities to summon and befriend Demons.
The gameplay is smooth, it's fairly easy to control your character and your Demon at the same time.
The sound... I never listen to the sound, I just listen to Winamp in the background.
All around the game is really entertaining, and I give it a 9/10 score, because it's free and it's a great game. The customization possibilities of your character is awesome. And this is how a MMO should be like. Sure you can spend some money on special gear.