I've played Realm of the Mad God since early 2011. If you're good enough at WASD dodging and you grasp the keys to progressing in the game--one that is permadeath--you'll learn to make quick decisions, balance risk:reward, and achieve great things. It's a source of hours, days, weeks, months of gameplay if you get into it. You can do it completely free (I've never bought weapons/armor), but in order to support the game I gladly bought vanity items (dyes/cloth, skins, etc.) and ease-of-use features (e.g. more character slots, vault chests for storage, etc.). The game is down for security maintenance now, but come join us when it's back up! It's certainly the best flash-based, in-browser top-down rogue-like-like twitch WASD dodge-n-shoot MMO I've ever played. ;)
RotMG is an awesome game. I was skeptical at first, but after giving it a go, I have played quite a bit of it. Haven't played in a couple years, but I think I am going to pick it up again.