Ive played a few mmos over the years uo, silkroad, age of conan, war, ryzom, entropia, guildwars. And played Eve with 30 days trial . Also tried and trialed loads of f2p and pay2p ones, but none captured my fancy for more then a week so i wont mention them at all.
From all mentioned above UO stands out before trammel introduction, unfortunately it went downhill from there, nuff said.
Saga of Ryzom (setting: mix between fantasy and scifi) an extremely good concept, a truly alive world (animals migrate with the weather and seasons, dig spots change. Crafting and digging(gathering) is one of the best ive encountered. Used to have one of the best comunitites around. Unfortunately due to bad advertising, and not sufficient development, it had to shut down servers twice and changed owners 3 times. New owner took over late 2008 but so far no sign of any development of the game (bug fixes, tweaks or anything at all) So ive susspended it untill there is something going towards new content or tweaks. If you are a new player id recomend trying it since there is alot to it. Graphics are pretty decent as well, concidering it was released just before wow did.
Entropia (setting: sci fi) is a fun and quite a good game, but due to the real money economy it is a huge money sink, this is also quite an atraction thought. The community is very mature, but due to the nature of real money involved there are Scammers (yeah with a capital "S") . Still its a free to try game and if you dont want to deposit any money you dont have to, but fun would be reduced 20% vs 80% grinding. Still worth a try since you dont have to pay a penny to try it.
WAR=fun untill you hit higher lvls.
Guildwars.....im not sure if i want to call it an mmo per se, more of a small groups coop pve, and some interesting battlerounds pvp, but it is quite fun.
Eve....was actually fairly fun but after trial run out i didnt feel the urge to subscribe, just not my cup of tea.
At the moment im wating for Darkfall and im actually surprised noone mentioned it here, since it is being developed by an indie developer. Game was supposed to be released on the 22/01/09 but got delayed, as mmo usually do just before the release, untill 25/02/09. The setting is fantasy, but at the moment its the only one that promisses a very unique gameplay experience. I wont go into details just check out their forums and website. Suffice it to say a lot of eve and old uo players are waiting for this game.
Also before anyone starts a flame war saying Darkfall is vaporware....yes it is in development from 2003, but everyone has their own opinions. Just google betaleaks for the game that is all.
Oh and just a small disclaimer, above opinions are just that. Opinions that are very biased and very subjective, if you disagree it is your right, but thats how i experienced abovementioned games. Hopefully this post will help people who are looking for slightly different mmo experience then wow clones.