What digital SLR accessories do I need?
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.
Good for: Everyone, but especially for news and sports, documentary/travel/wildlife, wedding and event photography.
It's a fact: digital cameras suck power, and digital SLRs suck more of it. A key component to good power management is having at least two batteries--one to shoot and one to charge. Having three is better: one to shoot, one fully charged spare on hand, and one in the charger.
Supplemental battery/vertical grip
Good for: News and sports, documentary, magazine/ street/ wildlife, wedding and event photography.
Larger, more expensive digital SLRs typically have a vertical grip with a second shutter release built in for taking vertically oriented shots. Many smaller cameras are compatible with separate vertical grips that incorporate an extra or larger battery.
Good for: Everyone.
Use UV or skylight filters to protect your lenses, and use polarizers to increase contrast, cut reflections, and deepen the blue of skies. Some filters that were useful with film -- color-correction and colored filters for black-and-white film -- can now be duplicated with digital settings. There are also many special filters available for creating effects such as soft focus, star-shaped flares, and gradients. Many special effects can be applied in image-editing software as well, but some photographers find it more convenient to use a filter instead.
Good for: Everyone.
Lens hoods minimize an optical effect called flare, which reduces the contrast and color saturation of your images and can create unintentional light artifacts in your images. No lens should be without a hood, and many come with them.
Good for: Studio/commercial, event photography.
Plug into the wall socket and forget all about exhausted batteries. Some sensor-cleaning methods require an AC adapter because it's a more reliable power source than batteries when you're holding the shutter open for extended periods of time.
Cable releases and remote triggers
Good for: Sports and news, documentary, magazine, wildlife, studio, commercial, event, fine-art photography.
A must-have for pretty much everyone, simple wired cable releases are great when you're using a tripod and want to fire your camera without shaking it. Fancier products, such as the PocketWizard and the Quantum Radio Slave, wirelessly trigger cameras and flashes from hundreds of feet away.
Good for: Sports and news, event photography, tabletop studio photography.
Though not yet common, many dSLRs from Olympus, Nikon, and Sony now have a transmitter built in. Add-on transmitters are predominantly available for professional Canon and Nikon digital SLRs. They're gadgets that mount to the bottom of the camera and lets you transfer images via either the 802.11b or the 802.11g (Wi-Fi) wireless networking protocol. While you're shooting in the middle of the bar mitzvah, your pictures could be popping up on a computer manned by your assistant at the edge of the banquet hall. He takes lucrative print orders from adoring grandmothers while you keep shooting.
Good for: All users shooting video.
The optical viewfinder isn't suited to shooting video, and holding the camera at arms length to view the LCD is both unstable and tiring. These gadgets, known either as LCD loupes or viewfinders, compensate for those inadequacies by allowing you to hold the camera up to your eye and see the complete LCD image.
Video shooting rig
Good for: Documentary, travel, or wildlife photographer; Wedding and event photographer.
dSLRs weren't designed to be used for shooting video, and a whole industry has grown up around kitting cameras out for that purpose. These Frankensteinian rigs come in various modular, user-configurable arrays, with components like LCD loupes, stabilizers, grips, and geared focus knobs.