I've been meaning for a while to put up a list of favourites, and since I've been trying (once again) to get my links and references organized, here they are, in no particular order:
PubMed. This is my one-stop shop, or at least a one-stop starting point for any research to do with medicine. With the pull-down menu on the left on "PubMed", I can search MedLine, the best known medical indexing service, for papers and abstracts. Also on the pull-down menu is "OMIM", On-line Mendelian Inheritance in Man. This began as a book on inherited disease published in 1982, and is now a massive, constantly updated database on inherited disorders. If there's a gene in the picture, it'll be in OMIM. "Books" covers a growing collection of medical and biology textbooks that are on-line.
The BBC. Particularly the Radio 4 arts and drama page, for a steady diet of radio plays. I love that medium. I have fantasies about getting to adapt one of my novels for it. Getting Ian Holm to play Lian, and James Earl Jones to play Rache.
Baen Books. Aside from publishing Lois McMaster Bujold and David Weber (and some other authors I enjoy), they put up lots and lots of words, including whole books, free!
Google of course, and a new search engine I was recently put on to, All the Web, which has, like Google, an image search and a news search.
The NASA website, and several of its many branches, including one on climate whose exact URL I cannot find at present, but that was very useful when I wanted to do warped and upsetting things with ocean currents and rapid climate oscillations.