OK, molecular biology sites:
The standby, NCBI : http://www.ncbi.nlm.nih.gov/
. This is the home of:
Pubmed : the journal search tool
entrez : an excellent nucleotide database, want to know the sequence of a gene? Find it here.
BLAST : nucleotide sequence comparison tools. Proteins, too. How similar is this gene to this one? Is there an analog of human gene x in dogs?
OMIM : mendelian inheritance in man. What does BRCA1 actually do to me? What genes increase my risk of heart disease? Find out here.
TaxBrowser : taxonomy database. Browse through the whole freakin' tree.
Structure : 3D structures of various proteins
....and a ton more stuff. It's a goldmine.
To note on Pubmed: You can output searches to an RSS feed. I love this. I have all my standard searches set up that way, so the current state of my field is staring me in the face as my homepage whenever I open my browser.
There is also Pubmed central. It's part of NCBI, but it deserves its own link, http://www.pubmedcentral.nih.gov/
. Many journals now put copies here, and anyone who does NIH funded research now has to copy all papers here. And it's public access. Fantastic resource.
Fantastic resource for biochemists. Lists out damn near everything by standard numerical classification. Gives all the info you could want, from inhibitors, affinities for substrates, expression patterns, species differences, the works. I love this one.
Google scholar : I'm sure you all know about this one, but for some searches, it's even better than pubmed. Especially older stuff. When looking for an older (per-1980) paper, you have a better chance of finding an electronic copy of the actual paper through Google scholar than you do through Pubmed.