Projectors are marvels of modern technology, but even more so when they're literally the size of a pack of cigarettes. The new Dell [DELL] M109S is only 3.6 inches long, 4 inches wide and 1.5 inches thick. It's not powerfully bright at only 55 lumens, and it's not quite HD at a resolution of 858x600, but it definitely gets the job done for the odd travel presentation.
Despite its small size, it has a throw distance of about 8 feet, and should be reasonably power-efficient with its LED lamp (although lest you be confused, this is not a battery powered unit; it actually uses the same power brick as many Dell portables, which is a nice touch.) The M109S is on sale now for about $500.