MegaMek is a Java version of BattleTech that you can play with your friends over the internet. All rules from Total Warfare are implemented. Rules from Tactical Operations and Strategic Operations are constantly in the works, and many of those rules and weapons are already implemented.
Players can create their own units, maps, and scenarios for use with MegaMek. MegaMek supports all unit types, from infantry, BattleMek, and vehicles, to Aerospace fighters, DropShips, and warships. Ground battles as well as space battles can be played.