The NASA Museum Alliance website provides informal education resources related to a growing variety of NASA missions. Registration is required.

An older (and no longer actively maintained) space science portal contains some information about space-science related projects and practices for informal educators.

NASA’s Planetary Photojournal is a great resource for high-resolution imagery of the solar system as seen by NASA missions.

NASA’s Human Spaceflight site provides a gallery of images and movie clips from Space Shuttle missions and previous human spaceflight missions. (You can find high resolution images of the Hubble Space Telescope here, as recorded in Servicing Missions.)

The Digital Image Collection at NASA’s Johnson Space Flight Center offers access to hundreds of thousands of images from human spaceflight missions and images of Earth from space.

Animators seeking texture maps of the solar system with accurate surface/atmospheric features will find this JPL site a treasure.

Two good sources for images of Earth are NASA’s Earth Observatory and Visible Earth sites.