1. The archetypal really stupid application for an embedded microprocessor controller; often used in comments that imply that a scheme is inappropriate technology (but see elevator controller). “DWIM for an assembler? That'd be as silly as running Unix on your toaster!”
2. A very, very dumb computer. “You could run this program on any dumb toaster.” See bitty box, Get a real computer!, toy, beige toaster.
3. A Macintosh, esp. a Mac in the original unitary case. Some hold that this is implied by sense 2.
4. A peripheral device. “I bought my box without toasters, but since then I've added two boards and a second disk drive.”
5. A specialized computer used as an appliance. See web toaster, video toaster.