The Encyclopedia of Diderot & d'Alembert. This magnum opus of the French Enlightenment took over twenty years to complete and includes 21 volumes with subjects ranging from asparagus to zodiac as well as a map of the system of human knowledge. They're still translating a bunch of articles, but if you're up to it, you can read it in the original French.
Bird Type Specimens. The Zoological Museum Amsterdam has put up 3D images of part of their extensive stuffed bird collection. Pigeon, owl, finch, bird of paradise...but alas, no chicken.
Pongomania! Wonderfully odd and surreal sculptures.
Alexander Graham Bell Notebooks Project. Liquid transmitters, plumbago experiments, and flame galvanometers. Now it makes me wish my poor lab notebook had way cool hand drawn diagrams too.
Early Office Museum. Lovely old doodads from copying machines and pencil sharpeners to key-driven calculators. And, of course, typewriters.
Twilit Grotto. An archive of grimoires and esoterica. Or at least it's a collection of the stuff that hasn't been burned a couple centuries ago.
AntWeb. Closeup images of the little beasties that will either elevate cuteness to another level or give you nightmares for the next month and a half.
Japanese Temple Geometry. During Japan's period of national seclusion (1639--1854), native mathematics thrived, as evidenced in sangaku--wooden tablets engraved with geometry problems hung under the roofs of shrines and temples. Also check out these sangaku problems.
An Ancient Greek Computer. "In 1901 divers working off the isle of Antikythera found the remains of a clocklike mechanism 2,000 years old. The mechanism now appears to have been a device for calculating the motions of stars and planets." There are also some simulations.
De Humani Corporis Fabrica. An atlas of the human body from the mid-1500s.
A Weekly Dose of Architecture. Former crack dens, ferry ports, floating restaurants, futuristic shacks...