McMaster: My favorite supply company for general engineering projects
Digikey: Comprehensive inventory of electronics parts
Jameco: Great electronics supplier
Surplus Center: Inexpensive source of parts for systems
SmallParts: Excellent source for project parts
American Science & Surplus: Inexpensive parts for projects
Harbour Freight: Source for imported, inexpensive tools
MSC Industrial Supply: Source for fabrication tooling
MPJA: Inexpensive electronics
Omnitronelectronics: Inexpensive electronics
US Plastics: Source for all sorts of plastics (raw materials, containers, . . . )
Parallax: Great source for hobby robots
Arduino: Open-source robotics platform
Make: Site dedicated to making stuff on your own