Organizations in the Washington, DC area with interesting activities
NCA - amateur organization with professional lecturers
USNO - rather technical colloquia; tour information
Wash Area Astronomers - professional but usually has a session for teachers
Exploring the Sky  - Monthly star gazing in Rock Creek Park

Great images
Astronomy Picture of the Day
JPL Image file
JPL Solar System Almanac
ARC image file
NASA image file
NSSDC Photogallery
Spacescience images
Galileo Images
NOAO Image Gallery
STScI

Other useful sites
Project 2061 - The AAAS education program and content guides
Naval Observatory tables - positions of planets, sun, bright stars, etc for one year
CLEA FTP
NASA Earth Observation Site -An extensive compilation of real data, theoretical exercises, pictures, demonstrations, etc.