An archive of protein structures determined by scientists using XrayCrystallography and other methods.

See:

http://www.rcsb.org/pdb/

Structures are identified by a code of the form Nxxx, where N is a digit (usually) and xxx is a three-letter code. For example, 1ird is a structure of human haemoglobin bound to carbon monoxide.

The RCSB site has options to view structures using VRML or an applet, but real scientists use specialised tools; to view PDB files in your web browser, get the chime plugin:

http://www.mdlchime.com/chime/