MegaMek is a freeware adaptation of the BattleTech rules that allows online and bot battles. It follows the rules from Total Warfare very closely and currently is adding the rules from Tactical Operations and Strategic Operations. Battleforce will eventually be implemented.
The game is Java-based, so works on all operating systems that support the Java Runtime Environment.
Players can create their own units, maps, and scenarios for use with MegaMek. There are also scenarios, map, and camo packs that can be found on various forums.
MegaMek supports all unit types, from infantry, battlemechs, and vehicles, to aerospace fighters, dropships, and warships. Ground battles as well as space battles can be played.
MegaMek is an open source project. People who are interested in contributing to the MegaMek project are encouraged to post patches, camo packs, scenarios, and bug reports to the MegaMek SourceForge page.