Good for: Everyone
Unless you're wedded to available-light photography, a good off-camera flash, especially one with a head that can twist, can be invaluable when you've got a borderline exposure situation. It can help eliminate shadows, add catchlights to the subject's eyes in a portrait, and freeze action when you don't want to jack up the shutter speed.

Sunshine in your camera bag.
Always carry a spare.
Grips like this one fit onto the SLR bottom.
Start your collection with a UV filter.
Block lens flare with a lens hood.
Most SLRs come with an AC adapter.
Cable releases attach to compatible shutter-release buttons.
A wireless transmitter typically attaches to the bottom of the SLR.
