7-Zip

7-Zip is the file compression software to get. Its main functions are to open, compress to and decompress from those odd RAR and ZIP files you occasionally run across and have no idea what to do with. For the more computer savvy person, it can also unpack every other major format (TAR, CAB, and ISO to name a few). On top of it all, it has its own compression format, named 7Z, which has an extremely powerful compression method of its own.

Portable version available at PortableApps.com