Terraria is a 2D mining game that is surprisingly fun to play alone or with a mid-sized group. One of my personal top 5 games on Steam.

Galactic Civilizations is fun and supports crazy huge maps in single player and large maps for multiplayer. ONLY recommended if it is on sale.

Sins of a Solar Empire: Rebellion is quite possible one of the best space RTS games made. The game is real time and supports up to 12 players.