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.

Futurlec has a very good selection of basic components, but they also have a wide range of ther things usually hard to find. Smart cards and readers, RFID tags and readers, unusal keyads, lots of good stuff. You should definitely check out their "Mini Board" modules which provide add on capabilities to micro-controllers for everything from MP3 players to stepper drivers to ehternet.

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.)