Where to buy parts, kits, & equipment
Sparkfun - Wide variety of sensors, Arduino-compatible components, open hardware specs for modifications
Pololu - Educational robotics kits, servos and controllers
Robot Shop - Professional robots for development and research, educational robotics kits
Acroname - Maker of the Brainstem microcontroller, also sells collection of sensors, controllers, and actuators for robotics projects
DIY Drones - The go-to place for everything UAV. Plenty of sensors and RF communications gear for all robotics
Maker Shed - All things Maker movement, including Arduinos and in-stock 3D printers
Jameco - Traditional distributor of discrete components, connectors, and tools for electronics
DigiKey - Same as above
Mouser - Same as above
Radioshack - Typically more expensive than the above but they have expanded their DIY sections in DC-area stores
AdaFruit - Like SparkFun, a good supplier of Arduino components.
ServoCity - Servo, robots, another RobotShop or Pololu.