Flashes
Almost all basic DSLRs come with a built in pop-up flash and they are adequate for snapshots indoors or possibly to fill in a bit outdoors. The problem is that they tend to look "flashy", are underpowered, and often, despite "red eye reduction" systems, hamper your image quality by red eye.
A solution is to get an accessory flash that mounts to the top of your camera. They are more powerful so you can shoot at a greater distance to your subject. They also have the ability to bounce the flash off walls and ceilings so that the images look less flashy. And since the flash mounts higher on top of the camera, it moves the flash out
of the plane of the lens and reduces the chances of red eye.
Sometimes, you can even use these flashes off camera - and this is where Nikon for the moment rules with their "Commander" mode. Although with the release of the Canon 7D, Canon seems to want to narrow that ground with a new off-camera flash system that I am sure will trickle down into their more budget models.