Tom Anderson has constructed a nifty list of online journals here. It also links to databases such as web of science and medline. There's an alternative list here - thanks to Oliver Holt for pointing out this one.
Throughout this site, I make use of entries from the biochemistry course web handbook.
The departmental powerpoint presentation archives. No, they're not complete, but they are getting better. Slowly.
Roll up, roll up, get your exam papers online (although they only have 2000 at the moment).
A guide to writing good exam essays is also available, provided by my tutor, Dr William James (ta!).
There's also some tips for surviving vivas.
Don't know a term? Try it at one of these dictionaries and glossaries. Note that the cell and molecular biology site also has link to various other online references.
Kimball's Biology Pages - an interesting attempt at a hypertextbook. Doesn't always go into as much detail as you might like, but can be useful to get an overview of a topic (and it has some good diagrams to boot).
This medical biochemistry page looks fairly comprehensive, although I haven't used it much.
A big 'ol list of Restriction Enzymes.
This page was written by Niall Harrison.