PartsSources

Motors, Surplus Electronics

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Smaller Distributors:

All Electronics
Hobby Engineering -- Sells amateur robotics tools, materials and supplies - including microcontrollers, development tools and semiconductors.
Spark Fun Electronics -- Another good amateur robotics and experimenter vendor. Also runs a small-run board production reseller that's not too bad for pricing. Excellent collection of SMD tutorials.

There's also a great chip search at [1] if you know what you want and are looking for prices and availability

Microchip Technologies -- Maker of the PIC line of 8 and 16 bit microcontrollers. Many of these have serial or USB capability. The have a free IDE and they have student versions of compilers made for their chips. There are also many open source projects relating to the PIC, including an IDE and a couple compilers. There is also a large web community, so any questions can be posted onto forums and answered by many experts. Don't buy from them directly, it's usually cheaper/easier to get <1000 pcs. from one of the vendors above under semiconductors.
Atmel -- Make another popular series of microcontrollers. These can be programmed in C using a port of GCC and have wide support. They are a manufacturer, and their website is great - but buy their parts from someone else, such as Mouser, Digikey, Newark In One, etc.
Parallax -- Makers of the basic stamp, which is easy to program, but a bit pricey. They also resell many common hobbyist microcontroller parts. Their prices are high on some items, but they also carry things that are uncommon elsewhere. Their new propeller chip also looks interesting.