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, especially the classic mac. 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".
This is not usually to be taken literally but, to show off the expansion capabilities of the risc pc, acorn computers ltd. built a seven-slice machine (which they called "the rocket-ship") and installed every imaginable peripheral. In a spare drive bay of the top slice they installed a toaster. This machine was exhibited at various shows where it attracted attention by occasionally ejecting a pizza.