Interesting articles on Biology and health topics from the Wellcome trust, good for background reading.
A textbook companion site with information, diagrams and quizzes arranged by topic
Part of the Open University site, has course materials at an introductory level which could provide interesting background reading on various topics.
Lots of Science News articles written for secondary age students, with links to original papers and glossaries provided for new vocabulary. Great for reading around the subject.
Interactive internet project – more than 1630 pages covering all groups of organisms
US revision notes style site – many topics covered with clear diagrams.