MegaMek is a conversion of the Classic Battletech boardgame allowing you to play against other players via hotseat or network. The program is designed for Battletech players already familiar with the game so you'll probably need to buy/borrow a Battletech manual. It covers the core rules and maps and the developers are working on including the optional rules. You can also create your own maps as well. I'm sure the Battletech gamers will love this.
For those of you who need to get introduced to Battletech and the glory of commanding giant mechs to blow each other up. Check out Classic Battletech. They even have free rules for new players!