Life Extraterrestrial: Life on Other Planets in the Solar System: Curriculum framework for comparing Earth to other planets with regard to life.
Search for ET!: Interactive Flash tutorial exploring the possibility of and search for life on other worlds.
The Planetary Society: A nonprofit organization devoted to the exploration of the solar system and the search for extraterrestrial life.
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.