Just a link-drop for those looking for a bit more advanced links:
Pubmed, aka Medline, should be an obvious one as most of our citations link to it; should be mentioned none-the-less
Scirus; a search engine that is not limited to peer-reviewed articles like Pubmed. Much wider breadth, although reliability of sources should be looked into if not from an academic journal.
PharmGKB, a database of prospective drug interactions.
Kyoto Encyclopedia of Genes and Genomes, a great site for flow charts such as Dopaminergic signalling cascades. They may look difficult to learn, because they are; however, these charts make the impossible somewhat feasible.