The Bad Astronomer
writes "Astronomers have used two big telescopes to create an infrared survey of the Milky Way that is the largest of its kind: the resulting image has an incredible 150,000 megapixels containing over a billion stars. Something that large is difficult to use, so they also made a pan-and-zoom version online which should keep you occupied for quite some time. These data will be used to better understand star formation in our Milky Way, and how far more distant galaxies and quasars behave."
The interactive image is powered by IIPImage
which happens to be Free Software and is cool in its own right (right click the image to get help — it has a full set of keybindings for navigation).