Setting aside "real" lives for the cause:

http://simcosmos.planetaclix.pt/, SimCosmos, perhaps better known as Antonio Maia.  Orbiter enthusiast and skilled add-on developer.

http://www.nasaspaceflight.com/, the amateur's information center on NASA's activities (often imitated, never duplicated...to the www.nasa.gov website like the Apple is to the PC.)  Founded by Chris Bergin, dwarfs After Columbia.

http://www.dunnspace.com , I think this really is his "real" life; come here for information on how to build economical single-use launch vehicles (in reality, or in Orbiter.)

The Da Vinci Project: http://www.davinciproject.com, This is an amateur project because it is staffed by a volunteer crew.  The Da Vinci Project is an X-prise contender and thankfully, has provided a prodigious amount of technical information on their spacecraft.

Space Island Group: http://www.spaceislandgroup.com, surprisingly similar to After Columbia Project, only bigger, doing for commerce what After Columbia is doing for amateurs and sims.

Encyclopedia Astronautica, by Mark Wade: http://www.astronautix.com; get up to date on everything here.

Martin Schweiger's Orbiter: http://www.medphys.ucl.ac.uk/~martins/orbit/orbit.html; http://www.orbitersim.com.

NASA Watch, of Reston Communications, run by Keith Cowing: www.nasawatch.com

Project Colimbo was trying to build a launch vehicle: http://www.colimbo.org/ (dead link)

PERMANENT for Projects to Employ the Resources of the Moon and Asteroids in the Near Term: http://www.permanent.com

United Space Development Team (USDT) http://www.unitedspacedev.org/ (dead link)

Chris Valentine's Columbia page: http://www.chrisvalentines.com/sts107 (has built in music, hit ESC to cancel music once you're there.)

OMWorld Consulting: http://www.io.com/~o_m; they have a very good FAQ on STS-107.

Universal Expeditions Project by Geir Lanesskog: http://www.geir.org/projects/uxp/

Columbia's Sacrifice: http://www.columbiassacrifice.com; independent analyses of available evidence. (dead link)

Fellow amateur Mike Majeski: http://gladiator1332.tripod.com : Delta Sprint's older brother? Big Gemini