Electronics

• adafruit is a great source for Arduino kits, sensors, components and kits
• SparkFun electronics has a vast selection of components for hobbyists
• Evil Mad Science specializes in LEDs among other things
• Industrial ARMworks ARM dev boards
• Newhaven Display sells inexpensive LCD displays
• zeroG Wireless makes WiFi modules for microcontrollers. Low power, low RAM, simple interface. http://www.zerogwireless.com/applications/appsconsumer.html

Mechanical

• SmallParts has a lot of mechanical parts
• McMaster-Carr has it all
• contraptor is open source erector set hardware for hackers
• https://sdp-si.com has crazy variety of couplings, belts, gears, etc. for industrial uses

Robotics

• The Robot Marketplace lives up to its name. Huge selection of parts but focus on R/C controlled robots, no CPUs here.
• RobotShop has tons of parts with more of a focus on microcontrollers and sensors. Seems to be part of Jameco?
• Lynx Motion makes many things but their Servo Erector Set is very nice.
• DU-BRO has a robotics section of their store with axles, bearings, wheels, etc.

Plastics & Moldmaking

• TAP Plastics has retail stores in Mountain View and other bay area locations. Good source for acrylic, resins, moldmaking materials, etc.
• Smooth-On has lots of moldmaking and casting materials including some low-viscosity line of resins. They have a terrific set of YouTube videos as well.

Home Automation

• Insteon network controllable switches
• DLI makes inexpensive web connected relays and power outlets
• Global Cache GC-100 web connected IR/IP/Serial gateways with switch sensors