The Rise and Fall of Star Faring Civilizations in Our Own Galaxy
Speculation, based on the Drake Equation, regarding the existence and composition of alien civilizations.
http://kurellian.tripod.com/ctcta.html

The Planetary Society
A nonprofit organization devoted to the exploration of the solar system and the search for extraterrestrial life.
http://www.planetary.org/

Life Extraterrestrial: Life on Other Planets in the Solar System
Curriculum framework for comparing Earth to other planets with regard to life.
http://www.resa.net/nasa/

Search for ET!
Interactive Flash tutorial exploring the possibility of and search for life on other worlds.
http://jvsc.jst.go.jp/universe/et_e/index_e.htm