I'm developing a browser game in HTML5, something like OSM, TribalWars, Khanwars, FarmVille...

I'm trying to find and already developed engine. Does anyone knows of any engine, made to build these types of games?

My thanks in advanced

If I recall correctly Game Maker exports to HTML5, that's an engine

If HTML5 is not a hard constraint, you can also use Unity to create Webplayer builds.

Proprietary player like Flash, ugly, I know.

With Unity 5 though you get WebGL Support.

Pixi is interesting. You'll need a little sound engine with it as it's only for video. Great tutorial here:

http://www.yeahbutisitflash.com/?p=5226