Online Suppliers of Electronics

Retail

1. http://www.digikey.com: Excellent search engine and documentation
2. http://www.mouser.com
3. http://www.radioshack.com: Always nice, since you can run to the local store for some stuff.

Surplus

1. http://www.allelectronics.com: Excellent selection
2. http://www.bgmicro.com: More digital electronics stuff. Fun catalog.
3. http://www.goldmine-elec-products.com
4. Bill Beaty's Surplus Sources List
5. HSC Electronic Supply: Various components, plus odd surplus and industrial stuff.
6. Alltronics: Surplus components and stuff.
7. Spark Fun Electronics

Cables and Connectors

1. http://www.sfcable.com

Hobbyist stuff

1. http://www.acroname.com: Nice collection of sensors, motors etc. intended for hobbyist roboticsors
2. http://www.parallax.com: Makers of the BASIC STAMP, a popular platform for hobbyists.
3. http://www.solarbotics.com/: Motors and small robotics parts, focusing on low-power applications.
4. http://www.mondotronics.com/: Home of "muscle wires" that contract when current flows through them.
5. http://www.robotstore.com/: Motors and robotics parts, including retail "muscle wires."