(For completeness sake, here goes...)
Default extension bz2.
Usage (simplest):
  • bzip2 <filenames>
  • bunzip2 <filenames>

Other commands: bzcat, bzip2recover.
Author: Julian Seward (jseward@acm.org)
Homepage: http://sources.redhat.com/bzip2
bzip2 is Open Source software, and has been ported to most common OS platforms. See the homepage for more info.