Robot Kits, PCBs, and Brain Boards

Solarbotics Kits/PCBs for Sandwich, Roundabout, solar robots (BEAM), and mini-sumo!
Wright Hobbies Robotics A variety of robots, kits, and parts.
Parallax Popular for their Basic Stamp line of products, including the Boe-Bot.
Active Robots A variety of interesting kits and parts in the UK.
Robot Marketplace Their small motors are in the Antweight section. They also have carbon fiber.
SparkFun Electronics Powerful module boards. Also, write ups of unusual projects and tutorials.
RobotShop Huge variety of robot-related stuff -- including motors, displays, and robot kits.

Electronics Components, Test Equipment, and Surplus Parts

DigiKey Large stock of electronics, easy to order, and reliable.
Mouser Electronics Good selection getting better all the time. Many unique items. Some better prices.
Electronic Goldmine Variety includes solar panels, switches, LEDs, and toy motors.
Electronix Express The least expensive source of most electronic parts, multimeters, and soldering tools.
All Electronics Overstock and salvaged items.
Future Electronics Not easy to order from, but carries some unique parts.
Jameco Electronics Lots of electronics. Some unique robot elements.
Circuit Specialists/WebTronics Good selection of meters. Nice wireless camera.
B.G. Micro Lots of interesting components, surplus items, small motors, and optics.
FindChips Great site for locating a chip seller.

McMaster-Carr Huge selection of materials and tools with helpful selection information.
US Plastic Low prices. Select "Sheet, Rod, Shapes" from pull-down menu.
MSC Industrial Supply / MSC Direct.com Similar to McMaster-Carr. Everything you can think of.
Micro-Mark Wide variety of small tools, including milling machines.
Brick Link Market of sellers of Lego bricks. A great way to get bulk Lego or missing Lego pieces.
Fortal.biz Sells chunks of Fortal aluminum (high-quality grade alloy) to hobbyists.
Little Machine Shop Tools, parts, and kits for milling machines and lathes.

Atmel Manufacturer of my current favorite microcontrollers, the AVR 8-bit RISC family.
AVR Freaks An independent site for AVR developers.
ImageCraft Development environment and C compiler for the AVR and other microcontrollers.
Texas Instruments A wide variety of logic chips, interface chips, and voltage regulators.
International Rectifier Some of my favorite power MOSFETs for H-bridges.
Fairchild Good MOSFETs and diodes as well as logic and interface chips.
Freescale My previous favorite microcontroller, the 68HC908.
Maxim / Dallas Another good semiconductor manufacturer.
TAOS Very cool light sensors! Some products are available at Mouser Electronics.
Ramtron Excellent serial memory chips! Some products are available at Mouser Electronics.
Energizer Specifications for the sizes, capacity, chemistry, and voltage of their power cells and batteries.
Duracell Technical data for their batteries (alkaline, NiMH, NiCd, silver-oxide, AA, AAA, C, D, button cell, and so on).

ExpressPCB Excellent quality. Good price on 3 prototype PCBs. Free PCB layout software.

Maxon High-quality miniature DC motors. The cost of brand new motors are out of the price range of most hobbyists. So, use this site to look up the motor specifications that are listed on eBay.
Faulhaber Makers of fine quality gearmotors with company names of "MicroMo", "Mini-Motor", and "Faulhaber".
Portescap Another manufacturer of excellent small motors. Sometimes the name is shortened to "escap".

Recommended Books on Robotics These are my personal favorite robot books.
Amazon Of course, Amazon has a ton of other engineering and computer science books.
Make Instructions and tutorials on making a wide variety of things. Always something new!
SlashDot News for nerds!
Invention & Technology One of my favorite magazines. Contains articles on the development of products and technologies in the last century.
Home Shop Machinist Projects and reading material for hobbyist machinists. The ads are useful for finding bargains and tools you never knew existed.
Apress The publisher of Robot Building for Beginners, Intermediate Robot Building, as well as a lot of books on programming languages.

Steve Hassenplug Steve is a master Lego builder. His creations and devices often outshine machined competitors. A friendly, great guy.
Hack-A-Day Constantly something new about reverse-engineering, hacking, or tweaking technology.
Society Of Robots Site and member-authored tutorials on parts, techniques, and getting started with robots.
Let's Make Robots Collection of member-authored blogs and articles regarding their robots, contests, or tips.
Instructables User-generated articles across many topics, including electronics, art, and robots.
Dave Hylands A well-respected robot builder and machinist. Lots of photographs and articles.
Taylor Deiaco A gifted young man who has documented his hints and tips for people starting out.
Alphadrome Vintage tin robots and space toys. I think the Robot Room mascot is a Nomura Zoomer Rachet.
Theodore Gray's Periodic Table A physicists dream: an actual physical table containing chemical elements. Fascinating short stories and pictures.