Free Digital Versions of Table Top Games

Vassal
Board Game Software (Play almost all games)
http://www.vassalengine.org/

Board Game Arena
Board Game Website (Play many euro games on this website)
http://en.boardgamearena.com/

Magic Workstation
Magic the Gathering (for real, all cards free)
http://www.magicworkstation.com/

Megamek / Mekwars
Battletech:
http://megamek.info/downloads