Almost every lens has threads on the front for screwing in filters; and there are lots of different types of filters for different purposes. "Clear" or "UV" filters will not have any visible effect on your images and can be kept on your lens permanently, so they are perfect lens-protectors. Your lens cap can fit right onto the filter when you aren't shooting.
Filters like this can be purchased quite inexpensively. You can get this one, for about $5 to fit the Nikon 18-55mm and Nikon 55-200mm f/4-5.6 VRlenses I recommend. If you end up scratching the filter, you can replace it for under $10 instead of replacing your lens for a couple hundred dollars.