It is, of course, free to play and runs entirely in a browser, which in itself is enough of a feat to warrant some gentle hand-clapping, but the fact the thing actually plays well makes it worthy of a standing ovation.
The basic premise is that your little character is dropped into a lovely 8-bit world populated by other humans, and you can run around shooting whatever you please, join people on quests or just careen around blasting monsters and looting corpses.
You'll start out by choosing from one of thirteen different character classes, all of which have their own assets and liabilities. From there, you'll be dropped into a safe area where you can wander around, talk to other players or just dive right into an area to get to killing. You'll start with just one class option then the rest will unlock over time. Control is handled by the WASD keys for movement, and the mouse controls shooting.
Along your journey you'll run into bosses, battles, tons of loot, different goals, pets and, of course, other people. What you decide to do with your time here is wide open, so even if you want to just pop in and shoot some crap for a few minutes you'll be just fine. There is, however, a permadeath feature -- so once you're dead, you're dead and you have start over. This likely ensures that nobody gets too all-powerful, but it also makes it so you're a bit more careful in your journeys.There are is a whole set of features we're not going to go into, but all you really need to know is that Realm of the Mad God is an incredibly versatile game that is worth the time it takes to get into it. Once you do, however, you may very well never escape.
Play it now.