Hobby Lobby: Lots of stuff for electric airplanes there. Best source for Czech Brushless motors such as PJS, AXI, Model Motors.
Balsa Products: the best source for GWS airplane products and parts. Excellent source for those nice/cheap Chinese-made Feigao brushless motors.
North-East Sailplanes: brushless motors, particularly the Rotex and MicRotex, AXI, etc. EPP airplane kits.
Aeromicro: lots of brushless motors (Feigao, Himax, AXI, Astro, E-Flight, Kontronic, Razor).
Todd's Models: A nice source for unusual and hard-to-find things, like contra-rotating propeller pairs, LensRC and LittleScreamers micro outrunner motors.
FMA Direct: a manufacturer/store for receivers, servos, autopilots. Official importers of Kokam Lipo batteries.
Hobby People: A huge on-line store, and one of the only places to carry the Cirrus Micro-Joule super tiny R/C flightpack.
E-Dogfight: nice little store for electric planes. Himax motors.
All-E-RC: lots of unusual brushless motors (Mamba, Hyperion, R-CER), including parts for do-it-yourself outrunner and CDROM-type motors. They also carry carbon tubes and rods.
Soaring USA: brushless motors (Rotex, MicRotex, AXI, Himax, Aveox)
Ikarus: An international on-line store with sites in Germany, France, and the USA. They carry the best European electric R/C equipment, like the Gigatronic flight processor, and the Aerofly flight simulator.
Esprit Models: Lipo batteries (ThunderPower, Kokam, Multiplex), Brushless motors (AXI, HiMaxx, Hacker, Razor), kits and ARF.
Dymond Modelsport: they have their own brand of servos and Lithium-Polymer batteries (4.4g servo for $20), and a nice array of brushless motors.
RC Direct: one of the few in the US to carry the Schulze battery chargers (the world's best IMHO).
Horizon Hobby: They used to be the best source for GWS products, but you can find that stuff anywhere now. Carry ThunderPower Lipo batteries.
Quantum Models: lots of things on this rather disorganized website.
Maxx Products: electric plane equipment. I don't understand why they still insist on putting their logo and using their own names on widely available GWS products.
RC Specialties: Depron, carbon tubes/rods.
Hobbytron: Toy-oriented on-line store that has R/C and hobby electronics stuff.

R/C Micro-flight Specialists

Bob Selman Design: magnetic actuators, Didel motors, JMP radio equipment, Blenderm tape. The best source for the smallest of the smallest.
Dynamics Unlimited: magnetic actuators, RFFS micro radio system, Didel motors, 2 and 3mm Depron.
Homefly: Lots of things that are hard to find from other places: actuators, 2.0g WES-Technik servos, Didel Motors, metric plastic gears, low-voltage brushless ESC, bidirectional micro-ESC... (no credit card. PayPal only).
E-Flight Design: actuators, 3.9g Blue Arrow servos for $17, E-Tec Lipo batteries, Didel motors, carbon tubes, Blenderm tape,...
RoomFlight: Didel pager motors and gear/drives, ultra-light props (including contra-rotating pairs).

Building Materials

Air Dynamics: Gabe Baltaian's great source for carbon fiber, Blenderm tape, Zepron, and more.
Depron USA: good north-American source for Depron sheets and carbon fiber tubes/rods.
RC Foam: another good north-American source for Depron, EPP, BlueCor, Arcel (EPP/ploystyrene mixture), RCF (like Depron, but denser), CoroPlast (corrugated plastic) and carbon fiber tubes/rods.
Aerospace Composite Products: carbon fiber and such.
Small Parts Inc.: every stock material and parts imaginable (screws, Delrin, Aluminium, Lexan,...).
RTL Fasteners: good source for small metric screws and such.
Aerocomposite Design: depron and such.

Radio Equipment, Electronics, Batteries

Castle Creations: Makers of the most popular Phoenix series of programmable speed controls for brushless motors, and the Pixie-7P speed controls for small brushed motors.
CheapBatteryPacks: best source for I-RATE Lipo cells and packs.
Batteries America: Platinum Polymer Lipo batteries.
LightFlightRC: primarily a Lipo battery vendor (Kokam, E-Tec, Poly-Quest, Thunder Power), but also brushless motors (Feigao, Himax, Razor).
Servo City: high-end servos, gear sets, torque-enhancing kits.

Micro-Helicopters

HeliHobby: lots of electric micro-helis, with parts.

Magazines and Portals to R/C Community Websites

RC Microflight: The premier magazine for micro R/C flying (subscription required).
E-Zone Magazine: best community web site for electric flying.
RC Groups: large discussion forum.
RC Universe: A bit disorganized. good for classified ads.

Manufacturers

Some of those have on-line stores.

RC Micro Flight

GWS: Grand Wing Servo, the Taiwanese manufacturer that made indoor R/C flying affordable. Motors/gears, propellers, ducted fans, cheap radios, small gyros, kits.
Didel: Swiss manufacturer of micro RC and robotics equipment: tiny pager motors, props, infrared remote control systems, actuators, gears and parts. Everything you need for RC airplanes below 10 grams.

Motors

Little Screamers: They make a nice little outrunner motor that weighs 23 grams, and can turn a 9x5 prop on 2 Lipo cells, or an 8x4.3 on 3 cells with a max current draw of 10 Amps. You can order direct with PayPal, or order from Todd's Models (see link above).
GoBrushless.com: All the parts for "do-it-yourself" outrunner brushless motors.
PJS: Pavel Jelinek's Motors: the Czech manufacturer of the PJS series of outrunner motors. From the 25g, 6.0Amps, 300g static thrust PJS-300 to the 730g, 50Amps, 8Kg thrust PJS-8000.
Model Motors: Czech manufacturer of the AXI and MiniAC brushless motors.
Hacker: US site of the German-designed Hacker brushless motors.
Free Air: site of the Czech maker of the HCS, Rotex, and Microtex outrunner motors. They also sell CNC machines for hobbyists.
Wema/Flyware: German makers of the Flyware outrunner motors.
Torcman: German makers of high performance outrunner motors.
Medusa Products: makers of the Afterburner brushless motors. The 12mm model can replace the GWS IPS motor (like the Feigao).
MotoCalc: not a motor manufacturer, but a software company called Capable Computing Inc. that produces a good performance prediction software for electric airplanes.

Electronics

Soaring Circuits: sell a 15gram, widget that records altitude for less than $100.
U-NAV: very light-weight automatic pilots with GPS waypoint navigation.
Schulze Elektronik Gmbh: German manufacturer of the world's best battery chargers.
Zlog: a cheap widget that records the altitude of your airplane.
Black Widow Audio/Video: nice set of wireless video equipment for model airplanes (including separate components).

Techniques

See How It Flies: an excellent book on the physics of flight by my old buddy John Denker. This book was written for full-scale airplane pilots, but it contains very valuable information about aerodynamics and airplane design.

Clubs and Individuals

Petter Muren's ProxyFlyer: A website dedicated to Petter's passively stable helicopter. In addition to all the R/C helis on his website, including the incredible 2.7g Nanoflyer, Petter designed the amazing Bladerunner toy helicopter.
AeroVironment: home of Matt Keenon, developer of the Black Widow micro UAV.

Lists provided by other sites

Vendors at the NEAT fair: electric plane vendors with a few microflight links.
R/C Microflight's Vendor List: links to advertizers and companies mentioned in RCMF articles.
Google's link directory for radio controled airplanes

Send comments and questions to Yann: