Spiral Knights is a browser-based, free-to-play, online mmo set in a fantasy / sci-fi world and is inspired by old rpg games like Zelda and Secret of Mana. While it’s an MMO, Spiral Knights also has a large focus on co-op, action RPG-style play. You can team up with up to four friends, and run around the constantly changing 3D world bashing monsters. And, of course, there are all the genre mainstays like character customization, weapon crafting, and guilds.
Their business plan seems to revolve around purchasing ‘energy’ which allows you to activate machines and purchase other things in game. This energy is also replaced every 24 hours but you can buy more at any time.