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.)

Xilinx-based FPGA (field programmable gate array) boards. High end development tools for special purpose solutions. They sell a range of FPGA boards and lots of add ons to do specific tasks.