Earth from Space - http://earth.jsc.nasa.gov
The best of NASA's Earth photography, with complete, searchable catalog data. Low and high resolution imagery. Frequently updated.

Gateway to Astronaut Photography of Earth - http://eol.jsc.nasa.gov/sseop
The entire collection of catalog records for the more than 400,000 photos of Earth taken during shuttle missions, most with low resolution imagery. A few high resolution images. Continuously updated.

NASA Image eXchange (NIX) - http://nix.nasa.gov
A search tool which can search imagery databases throughout NASA, tying together over 442,000 images.

NASA Human Space Flight Web, Multimedia Gallery - http://spaceflight.nasa.gov/gallery/
Press release photos from shuttle missions since STS-82, press release photos from the space station, and video, audio, and animation files from the shuttle and space station programs. Also contains some pre-shuttle multimedia. All new press release photos and other multimedia are published here. Low and high resolution imagery. Continuously updated.

KSC Shuttle Photo and Video Archive - http://science.ksc.nasa.gov/shuttle/photos
Preflight, launch, and landing photos and videos from shuttle missions since STS-67. Low and high resolution images available. Continuously updated.

GReat Images in NASA (GRiN) - http://grin.hq.nasa.gov
Over 1000 NASA images of historical interest. Images available in low, high, and very high resolutions - suitable for publishers and the media. Frequently updated.