BrowserQuest

In BrowserQuest players wander a tile-based top-down 2D series of grid maps depicting a gentle cartoonish fantasy land, with all interaction (upgrading equipment, conversing with NPCs, changing location and attacking enemies) handled by simple pointing and clicking. There are achievements to attain (killing x amount of rats, talking to Y NPCs, etc.) but the game's content is rather sparse.

Its presentation, however, is something else: the game is a MMORPG featuring teeming throngs of other human players running around teaming up on respawning monsters and poaching each others' treasures, and the whole thing is handled through open technology: HTML 5, JavaScript and introducing Web Sockets. This allows it to be played in most modern web browsers, on computers as well as on mobile iOS, Android and Firefox Tablet devices. Practicing what they preach, the game's source code is available to the public.