I was into Star Trek Online for a stretch. Played for about 6 months. Its about the same monthly cost as WoW. It's fun, but kinda dies off for endgame. I'm kinda waiting for it to become free-to-play.
Believe me, I have tried many MMOs, and none have the same 'charm' as WoW..
I played FlyFF before. It's fairly decent.. I don't think many play it now, though. It's kind of old.
I played Runescape too. I'd put runescape second to WoW.. I had great times in runescape, my friends played it and it has a good community.
Many despise it because of the graphics.. I don't think they ruin the game at all. You could always give it a shot.
I found Vindictus fun for a bit, but it's not very massive, it has but three classes as yet and they don't have many abilities, all but one or two missions are soloable, and there's an artificial limit to how much you can play unless you pay.
Lord of the Rings Online and Dungeons and Dragons Online are both free to play now. I played D&D Online for a bit after I stopped playing WoW and found it fun, but not my cup of tea.
I mean it's not crisp like WoW. You only have 4 races and they all look like something the Alliance threw up, the combat system is clunky and squishy-feeling, etc. etc. But it's charming in its own way, has pretty environments, and isn't too bad on its own as a diversion. The classes are fun to play.
EverQuest 2 is also F2P now but I can't personally recommend that one much. Bleh. You may like it, though.
Lord of the Rings Online all the way, I have a couple level capped characters there (Played long before it was free) And I can safely say that of the several MMOs ive tried, its the only one even CLOSE to WoW's awesomeness, not to mention its free now
Free MMos well Everquest 2 got a free option to it dont give all content but its well worth to give it a try other good mmo's
Everquest 1 best mmo ever made stopped playing it as sony tried to stiff me. other then that great game
Everquest 2 also great
AO not sure if its free to play but its awesome anyway
SWG not sure if its free to play still awesome game to play for the lore if nothing else.
DAoC also great pvp game not sure how active it is anymore tho.
some free games i know of
Ragnarok cracked servers
Flyff & Savage 2
If you just look for a little fun outside wow, while not raiding, or pvp'ing (or whatever your thing is), i can clearly recomment Lord of the Rings online. It has its own charm. The developers went alot into the storyline. So if you just take your time and read atleast the basics of the quest, then that will give you alot of fun.. Its free aswell
And best of all.. - Housing! - Which is something i personally think wow is missing.
Otherwise maybe try Guild Wars? Its kinda another gengre, but its pretty fun from time to time. And you can do much of it alone aswell
Im personaly looking at the Rift beta atm. It looks really really great, with all its new gameplay. Maybe you should go check it out. I predect its out in spring 2011
LoTRO is fun...if you are even a casual fan of LoTR (and by rights every WoW player should be) its worth checking out at the very least, as it is free now. I don't think I could ever get into too much stuff outside of questing though myself....combat in WoW feels much smoother. It is, however, very enjoyable if you just like wondering around questing at your own pace and absorbing a new interpretation of Middle-Earth....definetely feels like you are there. I'm sure there are other lesser known free ones that are also worth looking into, but personally I still enjoy WoW so I'm not actively looking, nor do I have time for another.