If you have an SLR camera that accepts interchangeable lenses, then buying a new lens could be considered an essential accessory. A new lens can give you a fresh viewpoint. In addition to the obvious choices of zooms and prime (fixed focal length) lenses, there are other factors to consider with digital SLRs in determining the right lens for the type of images you want to create.
For cameras that don't have interchangeable lenses, you can often buy accessory lenses that attach to the front of the existing, fixed lens. Although these do not offer high optical quality in comparison to SLR lenses, they do allow you to explore different fields of view, from telephoto and magnified close-ups to the superwide angle of a fish-eye lens.