kits

Images SI

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

hobby engineering

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

DonTronics

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.)

Electronix Express

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."

Golab Electronic Kits and Components

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