Since there seems to have been a flurry of interest in Mechwarrior and Mechcommander, I figured that I should make a seperate thread for the (Classic) Battletech board game.
Megamek is an online client used to play Battletech by many a folk. Very, very polished and complete (though it's constantly being updated) All you need is java to run it.
http://sourceforge.net/projects/megamek/Mekwars is a megamek campaign client that has an active community built around a number of servers in different campaign settings, utilizing a territory and economy setting with forces that you keep and gain XP with, etc. Very much like Europe in Ruins, ala Battletech.
http://www.mekwars.org/The main mekwars page has the client for a 3025 campaign server, which is a good place for beginners with basic level 1 tech and succession wars-style setting.
The server tracker lists all of the other known mekwars campaign servers, some are underpopulated but others are popular. Currently, I'm playing on Mekwars: Jihad and TAC: Sphere in Flames.
http://www.mekwars.org/mwtracker.htmlAs far as general links go, there are two that should be noted. The first of which is the official classic battletech site, which has very large and active forums, as well as being the place to ask questions and learn about the game (or catch up, if you've been out of it)
www.classicbattletech.comThe second link is to Battlecorps, the home of new (professional) fiction written within the battletech universe. You need to be a subscriber to get access to most of the stuff, but it's not expensive and very worthwhile if you're interested in reading more stories.
www.battlecorps.comLastly, if you want to read up on the rules, you're going to want to get Total Warfare, the new core rulebook which contains all tourney-legal rules for the game.