NOTE: The following is an historlic list or links. They may not all be up to date. I will possibly update them in teh near future.
Aero-Photo Hungary
http://www.aerialphotoshungary.com/
AeroQuad Open source multikopter
http://aeroquad.com/
AirFocus
http://airfocus.nl/
Antratek Electronics
http://www.antratek.nl/
ArduPilot
http://www.diydrones.com/notes/ArduPilot
ArduCopter
http://code.google.com/p/arducopter/
ARM-O-Kopter
http://www.armokopter.at
Ascending Technologies
http://www.asctec.de
Bernhard Konze’s hobby pages (including PPM to TWI conversion of ESCs)
Bernhard has been one of the drivers behind developing ways to convert commercial electronic speed controlers to be able to work with I2C or high rate PPM control needed for use in multikopters.
http://home.versanet.de/~b-konze/
Camkopter
http://www.camkopter.nl/
De Beeldenshop
…
CNC-Modellsport
http://www.cnc-modellsport.de/
DIY Drones
http://diydrones.com/
DM AeroPlatforms Ltd. Company
Develping a full-sized multikopter for search and rescue, in particular for rescuing people from high rise buildings.
http://dmaerosafe.freeservers.com/
FotoCopter
http://www.fotocopter.nl/
Futurlec, The Electronics Super Store.
http://www.futurlec.com/
Geopic
http://geopic.net/
Gibsonpix (Scott Gibson)
http://www.gibsonpix.com/
High Camera Hoogtefotografie
http://www.highcamera.nl/
Higherview
http://www.higherview.co.uk/
HobbyCity / HobbyKing
The best online shop for RC plane parts, including for multikopters.
Main warehouse in Hong Kong:
http://www.hobbyking.com/hobbyking/store/index.asp
German warehouse:
http://www.hobbyking.com/hobbyking/store/de.asp
Netherlands warehouse:
http://www.hobbyking.com/hobbyking/store/index.asp?whl=NED
Hoofddorpse Luchtvaart Club
http://www.dehlc.nl/
Jim Kearns quadrocopter pages (New Zealand)
…
LCC Shop (Chrsitan Linke’s shop)
http://www.lcc-shop.de/lcc-landing-page.htm
Luftbilder
http://www.luftbilder.immowelt.de/
mbed Rapid Prototyping for Microcontrollers
http://mbed.org/
Mikrokopter
One of the first groups developing affordable multikopters. The Mikrokopter was the first to really break through into the RC Aerial Photography arena.
http://www.mikrokopter.de/ucwiki/
Mikrokopter Shop
https://www.mikrocontroller.com/index.php?main_page=index&language=en
Modelbouwforum NL
http://www.modelbouwforum.nl/
Mr-Cam-Man
http://www.rc-cam.com/project1.htm
OpenPilot
http://www.openpilot.org/
Paparazzi: The free autopilot project.
http://paparazzi.enac.fr/wiki/Main_Page
QC Copter, Formerly the Big Quadro.
http://www.qc-copter.de/wiki/index.php?title=Hauptseite
Quadrocopter LLC, USA
http://www.quadrocopter.com/
QuadroUFO
http://www.quadroufo.com/
Quaduino-NG Low cost, Arduino based, multikopter project.
http://code.google.com/p/quaduino-ng/
RC Groups
http://www.rcgroups.com/forums/index.php
RC-Luftbilder.DE
http://rc-luftbilder.de/
Sparkfun Electronics
https://www.sparkfun.com/
Specky’s multicopter site: Luftbilder mit dem Quadro- und KoaxHexaKopter
http://www.fc00.de/
Skymixer
http://skymixer.net/
SkyShot NL
http://www.skyshot.nl/
UAVP-NG
http://ng.uavp.ch/moin/FrontPage
UAVX
http://multicopter.org/wiki/UAVX
http://code.google.com/p/uavp-mods/
XUFO Net Forum
http://forum.x-3d.de/bb/