Some really interesting sounding kits, unusual sensors, and other cool stuff. They seem to be aiming at the advanced science fair type of audience.

Robot kits, and robotics parts. Micro-controllers, and tools.

Located in Australia but they have a great reputation, and will ship to the US. The shipping is a little pricy though.

Enormous selection of micro-controller development boards and tools. Lots of cool add-on modules (xbee, USB, CNAbus, etc.)

Good test equipment site. Educational kits and the normal component selection.

"$20 minimum for merchandise, shipping extra. For orders less than $20, an additional handling fee may be charged."

Some very interesting kits. Lots of great PIR motion sensor kits.

Very large selection of parts. Many really good deals!