Web based game engines
- Open source is preferred to proprietary engines
- http://craftyjs.com/ - looks promising but is mostly focused on single player non-persistent one-shot games
- http://www.effectgames.com/effect/ - nice demos, but appears to be fully managed on their end. They expose a publishing platform, you develop your game using their toolset and on their servers. Similarly, all game data goes through their servers. Try out some of their demos though, it's very impressive.
- http://www.renderengine.com/demos.php - no current support for multiplayer, upcoming support planned using socket.io (already in use in http://vcweb.asu.edu)
- http://impactjs.com/ (proprietary)
Flash based game engines
- http://www.flashrealtime.com/flash-game-library-engine-list/ has compiled a list as well.