Determine whether or not you will ever want to replace your old light fixture before deciding on a method to cover the hole left in your ceiling after removing it. There are plenty of ways to hide the hole that still allow access to the electrical box in your ceiling, but if you are certain the box will never be used, a permanent patch may offer the best look for your home.
If you are not going to replace your old light fixture, cover the hole with a piece of drywall for a permanent fix. Use the same thickness of drywall as the surrounding ceiling, and cut the patch piece as close to the size of the hole as possible. You may need to nail an additional wood brace to the studs to give you something to screw the drywall piece onto. Once it is in position, caulk or spackle any gaps and then texture and paint the cover as necessary to match the rest of the ceiling.
Due to the popularity of pendant fixtures, many home improvement and lighting shops sell medallions to cover ceiling fixture holes. These medallions come in many styles -- make sure you choose a standard-to-pendant style -- and are made from a variety of materials. They mount to the electrical box or surrounding ceiling and cover the hole except for a small opening at the center, intended for a pendant cord. If you will not be replacing the ceiling light with a pendant, you can simply patch this small hole with painter's caulk and paint it to match the surrounding medallion.
Hide your old light fixture's hole with a relatively lightweight piece of art that can mount flat against the ceiling. Garden art, such as copper shapes meant to hang and catch the outdoor sunlight, work well in kitchens and bathrooms. A small painting tacked up on the ceiling can add interest to bedroom areas, where you are more likely to lie down and look upward. Use fasteners rated for the weight of any artwork you hang, and make sure to use anchors if fastening directly into the drywall rather than the wooden beams.
Scarves and other decorative fabrics can soften the look of a living room, bedroom or den, as well as hide unsightly fixture holes. Lightweight fabrics are easily fastened to the ceiling with thumbtacks or small nails, and you can select a color and style that matches your decor. Use caution when covering your ceiling lighting with fabric, however. Make sure all electrical wires are completely covered and held securely away from the fabric. Use only fabric that has been flame-treated or that is labeled as flame-resistant. Commercial window coverings, such as sashes and scarves, will meet flame retardant safety standards.