Fermi's Paradox
Links
The Drake Equation
http://www.seti.org/site/pp.asp?c=ktJ2J9MMIsE&b=179073
Describes the famous formula for estimating the number and character of alien civilizations within the galaxy. Includes an online form that calculates your numbers.
The Possibilities of FTL: Or Fermi's Paradox Reconsidered
http://www-personal.engin.umich.edu/~fritx/Ftlessay/essay.html
Discusses some of the logic behind arguments for and against humans being alone in the universe.
UFO Evidence : Fermi's Paradox
http://www.ufoevidence.org/topics/Fermi.htm
Links to articles and documents explaining or discussing the paradox.
Wikipedia - Fermi Paradox
http://en.wikipedia.org/wiki/Fermi_paradox
Encyclopedia article summarizing the concept.