Medirabbit - A guide to rabbit medicine, research and health problems
RWAF - Rabbit Welfare Association and Fund. The UK's largest pet rabbit organisation with a wealth of tips
The Rabbit Crossing - Facebook Notes page with some really good practical advice
Rabbit Rehome and the Rabbits United Forum - This site co-ordinates rabbits available for adoption via rescues, and has some good advice on rabbit care. It has a great and popular forum for anyone wanting to ask specific questions about rabbit care.
The Language of Lagomorphs - A guide to interpreting bunny behaviour and body language
The Rabbit House - full of useful information about rabbits, and a regularly updated blog, this site by Tamsin Stone, author of Understanding Your Rabbit's Habits, also includes a rather useful comparison chart of the different rabbit pellets.
H.A.R.E (Houserabbit Adoption, Rescue and Education) has some really useful articles too, especially in the Rabbit Health section.