A general biology site – click on articles for brief introductions to many A level topics (recent ones include lipids, carbohydrates etc.). There are also discussion forums and links arranged by topic area
US college textbook site – has interactive tutorials on various A level topics.
Health site (US) including multimedia virtual labs (cardiology / Elisa); animations (heart, bacteria, movement across membranes); click and learn (antibiotics, membranes, circulation, ECG); virtual museum (cardiology, senses)
Animations of various biological processes – colourful and easy to understand
Interactive tutorials, particularly on genetics and biochemistry
Textbook site with animations, quizzes and links