Adventure Quest Worlds - A 2D fantasy MMORPG that plays right inside your web browser. No downloads and free -- adventure along side other real players while battling monsters to obtain weapons, armor, pets and items in our new real-time combat system. Official site: http://www.aqworlds.com/
Wanted - The first browser based MMO based on Hollywood movie, I personally haven't tried it yet but I think it is special and deserve a try. at least for all you movie whore lol Official site: http://www.wantedthegame.com/Pros: +No download or install, plays right on browser. +Simple gameplay and user interface. +Multiclassing lets you mix play styles. +Large game world, instant teleportation via world map. +Constant updates and new features.
Cons: -Simple graphics, several visual bugs. -Some features are exclusive to paying members. -Quests reset when you log off. -Early level balance issues between classes.
zOMG! - a free 2D MMORPG currently in open beta testing. zOMG! is a new casual MMO that takes the millions-strong community of Gaia Online and brings it to life in the form of a huge virtual with plenty of interesting characters to meet and monsters to battle. Official site: Games::zOMG! | Gaia Online
Pros: +Connected with Gaia Online, shared avatars & items. +Original yet simple gameplay. +Huge player base. +Story/quest driven. +Great user interface.
Cons: -Simple graphics. -Too much quest text. -Appeal may be limited to existing Gaia users. -Tutorial locks in into starting area until quests are finished.