Telephoto Lens

Lenses that zoom longer than 50mm (in 35mm cameraspeak) are best for portrait work. They allow the photographer to fill the frame yet not get in the way. If you are standing too close it can be intrusive and this will be reflected in the picture result. Typically focal lengths of 85mm to 150mm are good for portraits.

Reflectors

A large reflector, like a white sheet, can be used to help 'bounce' window light into a room. Smaller, portable reflectors made from white card, are good for lightening dark shadows in contrasty sunlight or as a bounce board for a portable flash unit.

High ISO Setting

Though not strictly an extra piece of equipment, it's worth mentioning that your camera has variable light sensitivity. This is called the ISO value and is like having three different types of film in the same camera: slow for still life, medium for portraits and everyday snapping and a fast sports action film! Most cameras default to 'Auto', in which case the camera decides which 'film' it's going to give you, but you can also override this and set the values manually (in the camera menu).

Tripod/Monopod

Photographing indoors, even with a high ISO setting, can sometimes call for low shutter speeds, so a tripod or monopod can keep that camera steady and improve the chances of clear, sharp pictures.

Portable Flash Unit

If your camera has a hot shoe mount, an accessory flash or speedlight can be used to lighten up shadows outdoors or provide valuable extra illumination indoors.