I've recently read of and stumbled across a few great websites chock full of info, and I bet you there are tons more.  Everyone share your favorites.

BT Wiki: http://sarna.net/wiki/Main_Page (Duh)
Chaos March: http://www.chaosmarch.com/index.cfm (summed up stats)
Mech Ground: http://www.mechground.com/ (scanned read-outs)

Also:
Inner Sphere Cartography Society

http://iscs.teamspam.net/

If any of you happen to download the Battletech Giga-Torrent from the Pirate Bay, there are huge resources also related to geography (Battletech 1634, 1635 - NAIS 4th Succession War Militray Atlas Vols. 1 & 2).

While not as popular as "classic" battletch, I do find the Mechwarrior Dark Age stuff still entertaining if nothing else more than to hear what happened to my favorite characters.

I found the "Touring the Stars archive" to be the most informative.  It has information on the what happened to the various successor states and clans that had a presence in the IS after the World of Blake Jihad.

http://www.wizkidsgames.com/mechwarrior/mw_article.asp?cid=36765&frame=fanarea

Yeah I didn't like the small scaled focus they took with DA but the one thing that sounded interesting was the massive trials going on in the Clan Home Worlds.  With Ghost Bear, Nova Cat and Diamond Shark/Sea Wolf all relocating to the IS there's massive battle between the other clans to gain their old holdings.  Oh and lets not forget all the former Smoke Jaguar planets.

The other thing that surprised the heck out of me was the collapse of the Free Worlds League but the way they explained it made sense.