http://aplabs.org/ap2lab.htm
A very useful site with links to a whole range of topics in Anatomy and Physiology, it covers the main body systems. Not all sites are of the highest quality but a very good place to start.

http://blaufuss.org/
Interactive multimedia tutorials and tests are available for aortic and mitral
valve disease etc. Good animations, useful coverage of cardiac cycle, heart sounds and ECG abnormalities.

http://cochlea.eu/
An excellent website for the general public, students and professional scientists. It covers most topics relating to the ear and hearing. In English, French, Portuguese and Spanish.
 
http://www.ecglibrary.com/axis.html
An excellent resource with a wide variety of ECG traces illustrating many different pathologies.

http://www.endotext.org/
A very good resource for topics in clinical endocrinology. Registration is free and the various articles are well referenced.

http://www.heart.org/
The web site of the American Heart Association. A wide variety of information, mostly for the general public but has useful links to other health related web sites.

http://msjensen.cbs.umn.edu/webanatomy/
A site with online self-test material for basic anatomy of the major body systems.

http://www.biology.arizona.edu/human_bio/problem_sets/blood_types/Intro.html
A very helpful tutorial on blood groups, with some problems to test your knowledge.

http://www.cvphysiology.com/index.html
A web site covering clinical aspects of cardiovascular physiology, it includes topics such as heart failure, hypertension and oedema.

http://www.kidney.org/
The web site of the American National Kidney Foundation. Elementary discussion of renal function but useful information on renal disease.

http://www.renal.org/
The web site of the Renal Association of the UK. Useful information about clinical tests including a useful resource for urine microscopy.