There are plenty of sites containing some information on Mech Commander, however there are relatively few sites dedicated to Mech Commander. I've tried to post all sites that have information of interest on them and may also link to some sites containing Mech Warrior information relevant to Mech Commander once I get round to it. 

FASA Mech Commander Homepage - (http://www.mechcommander.com/)

This site has recently been replaced with a flash site for FASA studios on all their Battle Tech products and has little MechCommander Gold info. This link takes you to the right page - the direct link is: (http://microsoftgamesinsider.com/fasa/default.htm)

Mech Commander Strike Base Alpha - (http://web.utk.edu/~tone/mcsba/)

Drop Ship Command - (http://www.dropshipcommand.com/)

Unfortunately this site focuses more on the MechWarrior side of the Battletech universe and has less MC info.

MechCommander Unofficial Website - (http://members.tripod.com/~Mechcommander/gaz.htm)

A Battletech site with a Mech Commander section. 

Mech Commander 2 Homepage - (http://www.microsoft.com/games/mechcommander2/)

Mech Commander 2 Battlefields - (http://www.anzwers.org/trade/blaze224/)

Mech Commander 2 Mech Lab - (http://www.strategyplanet.com/mechcommander/)

This site mainly focuses on the editor and has a load of maps for you to download. 

Lego Mech Commander - (http://www.muenster.de/~bus1503/)

A site with loads of Lego Mechs'!

Sarna.net - (http://www.sarna.net/btech/bt-mc.shtml)

A small section on MechCommander 1 + 2 containing information etc...

Solaris 7 - (http://www.solaris7.com/)

Another site with little info on MechCommander, however the site is mainly dedicated to MechWarrior. It has some interesting Battletech info though.

Patrick's Battletech Archive - (http://www.kerensky.tierranet.com/btech/btech.shtml)

A site with a load of info on Battletech in general - historical stuff, all very interesting.