Pentax's PocketJet 3 Plus delivers 300-dots-per-inch resolution while weighing just over a pound with its internal battery in place. The PocketJet 3 Plus's thermal printing technology may not generate the highest-quality printouts, but you won't have to carry ink or toner around with you. (You will have to pack some thermal paper, however.) The PocketJet is designed to work with USB, and a Bluetooth version is available.
Photograph: Robert Cardin