http://megamek.info/
MegaMek is an unofficial, online version of the Classic BattleTech board game. MegaMek is open source, free software. It is licensed under the GPL.

We encourage you to download it and start up a few games with fellow members.

• Java based
  
• Network or hotseat play for 2 or more players
  
• AI opponent for offline play
  
• Mixed arms (Mechs, aerospace, infantry and other units)
  
• Many of the optional rules and technology from Tactical Operations, including advanced terrain and planetary conditions
  
• Map board selection and editing
  
• Mech selection and support for some mech editors