p7zip is the Mac version of the popular Windows decompression application 7-Zip. Note that it doesn't have a user interface - instead it operates only from a command line using Terminal.
However, there is a user interface provided by a separate program called Ez7z, which helps if you're not familiar with Terminal and command lines. p7zip allows you to create compressed files, add and remove files as a compressed package, unpack packages, view package contents, and even preview them to see if they are working properly.
The great thing is that p7zip supports a huge number of compression formats so it's very flexible. The obvious downside though is that it only runs using command lines. For more information on the available commands, simply type /Volumes/p7zip/7za in the Mac terminal.
p7zip is a powerful compression tool that is ideal for those who know how to use Terminal, but will prove tricky to understand by many due to the lack of user interface.