Human anatomy references, also the online home of Andrew Loomis' figure drawing books.
Thomas Suurland's homepage, with a metric assload of various blueprints of all kinds of vehicles.
Henry Gray's Anatomy of the Human Body. Plenty of in-depth stuff on muscles and internal organs.
Buildings, all sorts.
Mayang's online collection of free textures. If you have the spare cash, I recommend buying the collection on CD. Exceptionally good value.
ImageAfter, a good archive of assorted textures and reference photos.
I'll make this thread sticky for the time being, if you post a reply in it, please include a link to a site that hasn't already been linked in your post, to avoid cluttering the thread.