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.
PBS
Nova: Doomsday Asteroid Hale Bopp & Hyakutake
NOVA provides a report on comet Hale-Bopp and comet Hyakutake. Content includes discovery and observational information.
Other
Spacetech's Orrery: Comets Visitors From Afar
This site provides a report on comets, with many links to more detailed information. Content includes a focus on Halley's Comet, Hale Bopp, and Shoemaker-Levy 9.
Other
Cometography.com: C/1996 B2 (Hyakutake)
This site provides a detailed overview of the comet C/1996 B2 (Hyakutake). Content includes discovery, observational, and orbit information, as well as several images.
Other
Ulysses Space Probe Makes Surprise Trip Into Comet's Tail
An interesting article about a surprising finding on Ulysses' second orbit.
NASA
Nasa: Comet C 1996 B2 Hyakutake
This site from NASA examines all that is known about the comet Hyakutake. Select from "News Flash" or the table of contents located at the bottom of the page for specific information about the comet.