I haven't played this yet, but it seems to have a decent following on the classicbattletech.com forums so it must be doing something right.
www.megamek.net/
basically it appears to be a java version of the Battletech board game (for you console people, that's the game the Mechassault X-box games are based on). In my limited research it has online and hot seat modes. In looking through the files it also appears to have almost any mech you could want, and can import mechs made with the very nice Heavymetal Pro program. I have used Heavymetal Pro, even bought it even though I could have pirated it from my brother, and it is a great mech design program.
You will need a Java Virtual Machine installed, the documentation suggests java.sun.com/j2se/1.4/download.html
I'm going to give it a test run in a few minutes and will report back

edit: ok, the bat file wouldn't start it but double clicking the .jar file worked fine. played the lrm5 varient of the locust and a hunchback vs a battlemaster.  Battlemaster took some heavy damage from the AC20 but won the battle. I think I'm going to get my brother to get this so that we can continue playing BT since we haven't played much since I moved to Buffalo and we could easily play this