BrowserQuest is a cute, simple MMORPG made by Mozilla (the guys who make Firefox). It was created in HTML 5, so all you need is a modern browser – no plug-ins required. In BrowserQuest, you wander around the world and fight monsters to gain armor and weapons. There are funny pop culture references along the way (to things like Portal and Rickrolling) and there are 20 different achievements players can get. It’s a short enough game that you can complete it in under half an hour. If you’re wondering how it works, the source code is available on GitHub.
Play it now: BrowserQuest