(Yeah, I know it looks horrible. Sorry about that, but if I leave this offline until I get around to beautifying it it'll stay offline.)

Cats
The Cat Group  
Tanya's Feline CRF Information Centre Excellent support site for the owners of cats with kidney failure.
How to give subcutaneous fluids to a cat. A useful adjunct to the above - this gives a pictorial walkthrough of how to give subcut fluids to a cat.
Feline Kidney Diseases And Kidney Failure In Cats More information on the topic
Could my Cat have a FORL? FORLs are a painful tooth condition from which many cats suffer, sometimes with only subtle signs of discomfort.
Dr Addie's Feline Stomatitis page This page is designed for vets but its also the best site I've come across for clients.

Dogs
The Canine Heritage Breed Test Find out what breeds are contained in your Heinz 57

Exotics
Melissa Kaplan's Herp and Green Iguana Information Collection Extensive information on the captive care and behavior of green iguanas, and other reptiles and amphibians, by Melissa Kaplan, author of Iguanas for Dummies.
House Rabbit The Rabbit Welfare Association aims to improve the quality of life of pet rabbits in the UK.
Keeping Domestic Geese Keeping domestic geese: essential hints - useful if you have not kept geese before.
THAT DAMN BIRD! Parrot Psychology
RedTailBoas Community Forums Good site for owners of Red Tailed Boas.
Anatomy, Behavior & Diseases of Ferrets Hundreds of in-depth articles on the anatomy, grooming, behavior, reproduction, feeding, vaccination, parasites, infections, and other diseases of ferrets. Excellent informational resource for students, teachers, and ferret owners.

Charities
Blue Cross A charity helping small animals in the UK
PDSA A UK charity helping animals belonging to those on low incomes (defined as those receiving Housing Benefit or Council Tax Benefit)
RSPCA A charity helping small animals in the UK. Provides low cost (rather than free) treatment for those in its catchment area; inclusion criteria are broader than for the PDSA and include those on income support, disability benefit, state pension, working tax credit and jobseekers allowance.
Dogs' Trust A charity helping dogs in the UK (formerly NCDL). Dogs' trust has various projects including education, neutering, campaigning on welfare issues and advising dog owners undergoing a housing crisis.
Cats' Protection A charity helping cats in the UK concentrating on rehoming and neutering.