Whats a good MMO?

I presume you mean apart from wow?

Uh...
 
World of Warcraft
The Lord of the Rings Online
Aion: Tower of Eternity
Warhammer Online
Champions Online
City of Heroes
Ragnarok Online
EverQuest II

I hear Crimecraft is pretty good...

That's deep bro.

D+D goes free quite soon.

Guild Wars. It's awesome!
 
EDIT: Plus, no subscription fees! If that matters to you.

I played all MMOs on the market (I love MMos) and I gotta say I loved and got hooked to: 
Aion (went through all beta phases) 
Champions Online (went through beta and planning on buying game) 
WoW (I used to be an addict but i quit for good after getting 3 level 80s) 
Guild Wars (I still play it its amazing with all its expansioons) 
LOTRO (Amazing community and fun game specially if you love the lore of the lord of the rings) 
Warhammer Online (good community too, mainly ex-WoW players that love PvP)

Eve Online, by far the most indepth MMO going
 
If you are HARDCORE then Eve is for you.

A dead MMO.

I am not a big MMO guy, but I have been playing a free trial of World of Warcraft and it seems pretty good.  I don't really have anything to compare it to, but Blizzard knows how to keep their games going for years after their release

World of Warcraft for the usual mmo type gameplay
Guild Wars if you've played plenty of mmos and want something a little different.

Matrix Online
 
 
 
 
Oh wait.

See, a good MMO is sort of like...meat. Some people like Chicken, some people like beef, some like fish. And then there's the vegetarians who don't like meat at all.
It's all preference really. Do you like PVP, PVE, realistic graphics, cartoon graphics....

Aion: Tower of Eternity,  played a friends closed beta & will be getting it when it's released in North America in november.  Watch some of the beta vids on youtube, gorgeous graphics.  
 
WoW still tops my list of all time greatest MMO moments... but it will eat your soul (Mortal Kombat II Shang-Tsung style).  I absolutely refuse to check my /played.

There is no such thing as a good MMO. I like MMO's, none of them are good though.

" There is no such thing as a good MMO. I like MMO's, none of them are good though. "