Near-indispensible for image download and backup, as well as photo editing on the road (typically with Adobe Photoshop). The MacBook Pro shown is widely popular with travel pros.
Portable hard drive
Considered by many a must for redundant image backup on the road. The LaCie Rugged XL shown comes in 1TB and 2TB versions, and it has shock-resistant cladding.
Folding to a third their size, they provide portable portrait fill-in. The Lastolite 5-in-1 here has four different color reflectors, plus a diffusion scrim. Some pros just use trash bags.
Swiss Army knife
Handy tools: scissors for cutting duct tape, screwdrivers for various jobs, magnifier for checking sensor, saw for cutting most anything, and a corkscrew for relaxation after the shoot.
Silica gel packs
Essential in damp climates to discourage fungus in lenses and corrosion on mounts. Pack your gear with them in resealable bags.