Master Unit List: Also referred to as the MUL, this is a database of all BattleTech units of any kind ever mentioned in a (canonical) source. More information here.
Catalyst Game Labs: The official site for CBT's licensed publisher, which details their various gaming lines. It's most advantageous use is for retailers who stock CBT products.
CamoSpecs Online: The official site for unit paint schemes featuring hand painted miniatures displaying your favorite unit's battlefield colors. Open to submissions from the fans and regularly updated with new schemes most every week.
Catalyst Demonstration Team: The mission of the Demo Team is to muster a network of enthusiastic volunteers in support of Catalyst Game Labs (CGL) products, promoting awareness through in-store and convention demonstrations. Members of the team are called Agents, and are asked to coordinate their own local events as well as run global events organized by CGL.
Inner Sphere Atlas: an encyclopedia of BattleTech planets, with coordinates sometimes from canon sources, sometimes extrapolated from printed (canon) maps. Does not appear to have been updated in years, but provides quoted material about the planets and the information is still relevant.
Pryde Rock Industries: This site, operated by 'Medron Pryde,' is a large depository of accumulated data, including programs and fan-created material. He seeks out any aid for CBT use, often providing several different fan-created spreadsheets, rules and graphics. His editorials are often insightful. A good first start for someone first entering the CBT world, as well as the old MechWarriors. (last update April 2015)
Solaris 7: A community of CBT fans with online TRO's, both custom and canon, with a vast array of resources such as mech editors, a planetary database, an image gallery and active forums that hold regular design contests. Design and post your own custom BattleMechs, Aerospace Fighters, Vehicles or even Warships and get feedback from fellow designers. A large group of veterans are on hand to help new players get accustomed to the design process and to understand the CBT universe as a whole.
Wayne's Books FASA Battletech Reference: Descriptions, content lists and pictures of the original Battletech and Mechwarrior games sets and books published by FASA Corporation in the 80s and 90s.
BattleTech Engineer: A blog supporting various BattleTech computer applications, including the BattleTech Encyclopedia at http://goodsects.gotdns.com/bte which contains a full database of every single BattleTech unit, and will generate record sheets for each of them (last update Aug 2019)
Unit Color Compendium: A database and search engine for BattleTech unit color palettes and paint schemes.
Battletech-Mercenaries - An online RPG based on the Battletech Universe. Methods of play include PBF and PbEM. If you always wanted to write your fan-fic of a ruthless mercenary trying to make out his living in a Merc Unit, this is the place for you.