I play World of Warcraft.
I tried EVE Online for a week, but it didn't appeal to me. I just don't have time to get to the good stuff in that game.
I played a week in the closed beta for LotRO. I really liked it. If I didn't play WoW, I would likely play LotRO.

5 creepy ways MMORPGs are designed to get you addicted:
http://www.cracked.com/article_18461_5-creepy-ways-video-games-are-trying-to-get-you-addicted.html

/agreed
But I am finding good ways to waste my time until it is released with DDO and GW1, lawl.

Phew, I've played a holy hell of alot of MMO's.
EVE Online, World Of Warcraft, EverQuest 2, Guild Wars, City Of Heroes / Villians, Star Wars Galaxies, Warhammer Online, Anarchy Online, Asheron's Call 2, Dungeons & Dragons Online: Stormreach (was even part of the early beta back in day), Lord Of The Rings, The Matrix Online, Pirates Of The Burning Sea and this is not listing all the free ones I've played.
I am currently playing around with Champions Online, one of the few MMO's that I've so far really enjoyed. I am also really looking Star Wars: The Old Republic, being a huge Star Wars nut and such.

Currently, I am waiting on The Old Republic (so, I have around a year to wait).
I have played in the past (in no real order, except betas are last):
EverQuest
EverQuest 2
Pirates of the Burning Sea
Dark Age of Camelot
City of Heroes/Villains
Tabula Rasa
Auto Assault
Rappelz
Runescape
Jumpgate Beta
Neocron Beta
EVE Beta
I haven't really played an MMO in a couple of years. There have been few (none off the top of my head) BioWare games I have disliked, so I'll give The Old Republic a chance whenever they release it. If it allows me to be as vile and evil as Darth Sidious, I might even keep playing it for a while ;).

I've never played an MMO, I refuse to pay a subscription fee. But I did get Guild Wars Trilogy for $15 and am looking forward to giving it a shot now.

Currently have 5/7gigs of Everquest downloaded. Got that, the 15 expansions, and some items for $2.50 on Steam.
Oh, so we are posting games we have also played?
City of Heroes (Meh)
Beta tested Free Realms (Double BLEH!)
Beta tested Dragonica Online (Okay I guess.)
Florensia (GRIND)
Tales of Pirates (Torrented it from the site, fried my computer.)
Wonderland Online (Okayish.)
Wonderking (Meh.)
Maplestory (GRAW!! SUCKS HARD!!!)
Atlantica Online (Crafting sucked, translations were annoying, but fun)
Runes of Magic (You want a grind?)
WoW (Doesn't appeal to me)
Perfect World International (Okay, grind heavy, boring community.)
Gates of Andaron (Meh)
Bright Shadow (WTF???)
Shaiya (It was okay, just got to repetive.)
Wolfteam (Lag fest)
Crossfire (Became a ghost mode pro and quit)
Combat Arms (Boring as hell.)
Pirates of the Caribbean Online (Don't bother)
Wizard 101 (Play occasionally, gets repetitive towards level 25+)
Allods Online (Okayish)
D&D Online (Not missing much)
Fiesta (Don't even attempt to enjoy it, you wont.)
Flyff (Eww)

Oh, not that anyone cares, but I tried WAR. Couldn't really get into it. So probably not for me.

Well, Guild Wars isn't technically an MMO, but it might as well be, due to it being the only game in it's genre that didn't die, and there was only ever 1 other(Fury), so....
Guild Wars is a Competitive Online Role Playing Game(CORPG) basically meaning that the PvP is incredibly highly structured, and it's actually pretty fun if you get into it, but go through the PvE portion first to unlock some skills if you plan to PvP, it's pretty fun too, and heavily story-driven. And apparently they have something big planned for the end of April.

Dragonrealms!!! although that probably doesn't count.
I tend to avoid MMOs, they pure timesinks with no END. There's always the next item, the next level, the next boss to beat, but there's never any satisfying conclusion. I much prefer games with a start and a finish, plus if I actually started an MMO I feel like it would just get stuck in it

The genres are very similar, need to be online to play, play with other people, group up to quest, etc. The biggest difference is the PvP structure, a lot of the early focus was put into Guild versus Guild(tournament every day with a big one every month) and Heroes' Ascent(continuous tournament).

I used to play maplestory until it stopped working on my computer.
I still play mabinogi. Desertfire is my main character and my elf.

I've played WoW on and off since early beta, my interest has waned in it since the latest expansion hit though. Tried out Warhammer, AoC, and LotR when they came out but nothing really held my interest for that long.
I notice when I get drawn into an MMO that it completely dominates my playing time, and I miss other decent games when they get released, which is partially why I have such a massive backlog of unopened games. I want to try out star trek online but I'm waiting until I finish a few other games in case I get drawn into it.

Oh, come on. I'm also guessing you're around 13 or so.