NetBiochem
Based in Utah, this site has some helpful summary tables.
British Medical Journal
This link takes you to a premier medical journal that may provide useful information on medically-related topics.
DIY glycolysis
A well-constructed and helpful resource for understanding glycolysis. Written by Jon Maber at Leeds University.
Glossary of Biochemistry and Molecular Biology
An encyclopedia edited by David Glick and hosted by Portland Press, the publishing wing of the Biochemical Society. Useful for checking out any terminology you haven't understood.
Gram-positive or Gram-negative?
A good guide to microbial taxonomy. If you need to know whether certain bacteria are Gram +ve or -ve this site at the University of Texas could be the place to look.
Harrison's online
For the more medically-related topics, you may find some useful material from this online textbook.