There are many sites. Just do an AltaVista search under Comets. I have included links to some of the ones I use in some of the questions in my Meteors and Comets archive. NASA/JPL and the popular astronomy magazines have good links on comets.
A good introduction can be found at The Comet's Tail, and JPL has a great web page on Comet Hyakutake They also have a massive Comet Observations Page that has just about everything you need to know about past comets of note.