Circulatory System:

Video on Teacher Tube: Heart dissection
Great animation about how the heart works here. Video of heart and circulatory system here.
How Stuff Works - How Your Circulatory System Works
Human Anatomy Online - interactive diagrams of all the systems, including nervous, skeletal and reproductive.
KLB Science Interactivities have produced a clever quiz on the heart and circulatory system.
More great Human Body stuff from National Geographic here.
Virtual microscope images of the circulatory system from the Indiana University Bloomington.
Image Source - Note the five different types of white blood cells.
Components of blood
Virtual microscope slide of blood
Virtual microscope images of arteries, veins and capillaries showing tissue types.
What is Haemoglobin?
How is altitude training relevant to blood systems?
What is blood doping?

Respiratory System:

Respiration - University of Melbourne animation of lung structure showing alveoli.
Habits of the Heart - Lung Structure from the Science Museum of Minnesota.
Video of the Respiratory System
Human Anatomy Animations of the respiratory and circulatory sytems from Bioanime.
This site includes animations of all the human tissue types, including the different types of white blood cells.

Excretory System:

Great interactive animation showing structure and function of the kidneys.
The nephron - structure and function.
The Kidney Quiz from Zerobio.