You did not specify client based MMOs, so I will write which browser based ones I like:
http://www.UrbanDead.com - Zombies versus survivors. It's hard, I don't know how rewarding, but if you are in a good faction, then you will have lots of fun. Members of Abandonia's forums have created in the past the faction named
The Abandoned. While original members have left, me and Doubler are still there, along with many other people. Level limit 42.
http://www.NexusWar.com - Angels, Demons and plain people, in a fight for existance. It has lots of features, like spells, crafting, alchemy, pets, combat with swords, melee weapons, hand to hand etc. Updates aren't taking place often, but when they do, they are big. The Abandoned are also present here. Level limit hard to achieve: 30.
http://www.SWCombine.com - Star Wars browser game. In existence since 1998, with a huge community, friendly people, lots of roleplaying, and many features. Combat still isn't present, but it will be next year. NPCs, factions like the Trade Federation, Rebels, Galactic Empire, extensive rules, has many, if not all, Star Wars planets, ships, weapons and armor. No level limit, but leveling takes a high amount of time.
All three are free to play, with Combine needing a paid email address -from your ISP, for example, not Gmail, Hotmail, Yahoo etc. Donations are accepted by all of the games, but as a reward, you do not get items or abilities which can affect the game - no boosts, no special items, no new features. The above text might sound like an ad, because I like the games way too much :P