Wikimedia
Wikipedia: Comet
A nice encyclopedia article from Wikipedia on comets describes what they are, their physical characteristics, gives a history of comet study, and provides a list of well-known comets.
California Institute of Technology
Spitzer Science Center: Riding a Trail of Debris
This image, titled "Riding a Trail of Debris" features the comet Encke with a clearly visible tail comprised of "gravel-like debris." The text under the image details various specifics about the picture.