Minethings.com is a browser-based MMO that I've been running for a couple years.  A friend told me about bitcoins a couple weeks ago and now (after a weekend of coding) they are accepted at Minethings.com.  The game is free to try out and in-game purchases are optional.

A bit about the game:

A real-time mining game based in a post-apocalyptic Earth where there are no rules and everyone's loot is up for grabs. Inter-city trading is all PvP so it requires arming up your vehicles and being prepared for the worst. But the rewards are worth it!

Grow your first humble mining operation into an empire with equipment, mining robots, battery upgrades, and gadgets. Expand your operations to the high seas, build factories, earn interest on your wealth, dominate markets and climb the ranks to become the one with the most things.

Start mining today!