Cavy Madness Contains an informative message board.
Seagull's Guinea Pig Compendium is a fantastic source of information to help you care for your guinea pig.
Cavy Cages is another excellent place for information, specifically on how to house your guinea pig.
PetPlace.com contains helpful articles on such things like training your cavy.
Oxbow Pet Products a very good source for hard to find Timothy Hay.
Leith Petwerks another source for Timothy Hay.