this database has a huge ammount of games, set it to free if you want free games and make sure you set it to online unless you don't mind playing games that isn't online.
Ryzom - scifi-ish 3d opensource fantasy mmorpg http://ryzom.com/?page=page_features
Regnum Online - a free to play version of dark age of camelot is what it's mostly called, the graphics may look lower quality but the gameplay is very rich http://www.championsofregnum.com/index.php?l=1&sec=6
Runescape - could feel a little clunky and odd playing the game but some bosses in the game are real nice and once you understand how the game works, it gets really interesting and if you can play in max settings, it actually looks pretty good
i've hated this game for over 7 years just because it looked ugly back then and suddenly stupidly got hyped from a video from 7 years ago when it was just dumb numbers going on top of the characters lmao.. xD http://runescape.com/
Tibia - the graphics and the camera angle ticks me off so i haven't played but alot of people seem to love this game for who knows what, it looks silly but they did say the gameplay is good, not sure about it but its worth a try.
Wakfu - a few people mentioned that wakfu works on ubuntu, if it does this is the only final fantasy tactics - like strategy turnbased game ever, sad part is you can only control one character, honestly it doesn't concern me all i need is one character. the characters sadly look a bit goofy to keep me interested.
Wurm Online - It's Massively Multiplayer Online Fantasy Simulator (MMOFS) http://www.wurmonline.com/ i dont see much people mention this as much but i think it's still worth a mention http://www.wurmpedia.com/index.php?title=Basic_Knowledge
Vendetta Online - basically eve online a spaceship game but i think this one has smaller ships 8 hours trial but looks decent and alot of players seem to mention it.
Planeshift - a 3d opensource fantasy mmorpg, some people love it some people don't i found it pretty cool how players could participate in developing the game.
try this also if you don't mind using wine
my favorates on the list are
Aika - a 3d mmorpg that moves in wasd, decent graphics supports 100 vs 100 (or was it 300 vs 300?) faction wars
yet has such a small client.. the game sort of has genderlocked classes either way it's really fun questing.
Aura Kingdom - 3d fantasy anime mmorpg, it's one of the newest ones from that taiwan company, the graphics are so adorable but unlike most anime mmorpgs they look older, not 7-8 year old characters! like luna etc xD, i heard combat wasnt perfect, seems more like a graphic game but it's still popular.
Digimon Masters - yes digimon.. might sound childish but the game itself isn't as grindy, it may be annoying getting digimon eggs that you want but controlling pets is pretty rare, i think only a few tried that in an actual non-turnbased mmorpg. Dungeon Fighter Online - dungeon crawling 2d old style beat em up aka dungeon&runner remake, it's currently in alpha phase and it will end in june 13rd, the game is really fun when it comes to comboing, pvp is my main interest in this game.
you join the arena channel then join a room or make one and start when your opponents are ready and fight a free for all / elimination match where upto 8 players can fight each other 4 vs 4 one at a time or just do a team match 4 vs 4 right off bat. i can't wait for the official release of this game :P
Path of Exile - a diablo style game with many ways to build your character differently, the file size can get large i think it was like 6 gigabytes or more but the gameplay is worth it, though it needs a good computer or else your computer will bottleneck
s4league - third person shooting room based game with few other modes available, you can also use melee weapons and fly
walk over walls.
Tera Online - if you can run this.. in my opinion i would pretty much ignore whatever i just said because i can't resist the graphics this game has and the pvp servers being able to pk people at lv.15~20 and so many races, so many classe available, it has decent pve content.
Atlantica: if you like turnbased games, this 3d game is for you.alot of players like this game after all there arn't many turnbased games available.
Star Wars: Old Republic - do you like lightsabers and jedis? honestly im not a fan of this game because im not a starwars fan i mean i do like red lightsabers but the gameplay there don't interest me and most of the time i saw my friends just afk quest there, sending their characters on dispatch missions and just watching tv lols
i usually add screenshots as well but excuse me for not doing so, my ie crashed 12 times while trying to post this so i had to type it over and over and over again!!
A lot of the MMORPGs above listed need Wine to run correctly. I made a thread in the kitsu saga forums ages ago that detailed how to go about running X-Legend MMORPGs, but the forums for Kitsu Saga were all removed, so it's unlikely you'll be able to find it.
All the X-legends games run semi-fine in Linux, you just have to make sure to set the FPS to maximum, and have reflections turned off and any post-processing effects.