BackTrack is the most Top rated linux live distribution focused on penetration testing. With no installation whatsoever, the analysis platform is started directly from the CD-Rom and is fully accessible within minutes.
Currently BackTrack consists of more than 300 different up-to-date tools which are logically structured according to the work flow of security professionals. This structure allows even newcomers to find the related tools to a certain task to be accomplished. New technologies and testing techniques are merged into BackTrack as soon as possible to keep it up-to-date.
Check out the BackTrack wiki for more information on the project and tools.