Mining is basically my favourite thing to do in games, and in the last year I've been served well by Minecraft, Wurm Online, Terraria and, now, King Arthur's Gold. The multiplayer, team-based sidescroller is about two teams of Knights, Archers and Builders battling for control of a level's gold supply.
It's still in early beta, but already worth playing. Builders can mine through rock to find gold and build structures like ladders, doors and catapults, while Knights and Archers defend and lead attacks on the enemy's gold reserves. It's heavily inspired by King Arthur's World , a sidescrolling RTS on the SNES from 1993, but it plays more like a competitive Terraria.