I use to play WoW and runescape, but quit due to many 4-5 year olds crowding the servers. WoW hasn't gone down that road yet, and hopeful it won't. But I would recommend WoW.
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Pros:
Good graphics
Many players
Not to boring
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Cons
Expensive
You must download
I started off playing tibia is a fun game but very bad graphic... after that i played runescape. Then i branched off play like 5 or 6 different games all in the same week...
I play Mabinogi and sometimes Runescape. Maybe you can find me on SotNW(Sword of the New World), Wonderking, or SUN(Soul of the Ultimate Nation) though.
If you're interested, PM me and I'll give you my IGNs
I use to play a lot of mmos but most if not all of them were free. The entire list of any of them is on my xfire: http://www.xfire.com/profile/spy2brandon/
The only one I play now is EvE
I've played quite a few, being MapleStory the one i got higher.
Else than that, i've played Cabal, Ether Saga, Silkroad, Lost Saga, MU online, Deicide, Eudemons, etc etc etc.
Now im into EvE and it's great, so i've plans to keep going there and bomb some stuff.