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Probably my favorite electronic components vendor. Definitely check them first as they tend to have the best prices. They also have a new program called Club Jameco, which has all kinds of little project kits that people have invented that you can buy. I'm even thinking about putting some of the Techsplorers project kits onto Club Jameco.
A major online vendor of electronic components. They have some items that you can't get elsewhere.
Another major online component vendor with a very diverse product line including microcontroller development platforms from all of the major microcontroller vendors.