Planetary Mission Data and Images:

Planet (including moons)	Link	Description
Mercury	MESSENGER Gallery	Gallery of MESSENGER images and other info
Mars	JMARS	Java-based geospatial information system for Mars datasets
Mars	MOC Images	Theme index of MOC images with captions, organized by type of features seen (layers, gullies, volcanoes, etc.). (NOTE: scroll down past calendar index)
Mars	HiRISE images	Theme index of HiRISE images, organized by science theme (aeolian processes, glacial processes, mass-wasting processes, etc.)
Mars	THEMIS images	Theme index of THEMIS images, organized by topic (channels, landslides, craters, etc.)
Moon, Mars, Venus, Jupiter	USGS PIGWAD Maps	GIS-based interactive maps and geospatial information
all	NASA JPL Photojournal	Searchable collection of captioned images from all NASA planetary missions
all	NASA Image eXchange (NIX)	meta search engine for all of NASA's online image databases
all	NSSDC Photo Gallery
Jupiter	Galileo Legacy Site	Well-organized interface for browsing images of Jupiter's moons, including clickable maps for Io, Europa, and Callisto
Jupiter, Saturn, Uranus, Neptune	CICLOPS Search	Searchable database of captioned images from Cassini, Galileo, Voyager and New Horizons
Saturn	CICLOPS Maps	Mosaic and cartographic maps of Saturnian moons
Saturn	Cassini mission homepage	Official homepage of Cassini mission; press and raw images but inefficient interface

Journal Articles and Conference Abstracts:

SAO/NASA ADS	Searchable database of journal articles and abstracts
2009 Lunar and Planetary Science Conference	PDF of conference program with links to abstracts
Google Scholar	General search tool for papers and abstracts, with links to cited and referenced articles

Miscellaneous Links:

Alan Howard - Cratered Landscape Simulations