7-Zip

This probably isn't news, but a coworker just told me about 7-Zip. I've been a fan of WinRAR for my compression/decompression needs in the past, but 7-Zip is open source and freely downloadable. It doesn't have quite the full range of support and features that WinRAR has, but supports unarchiving of nearly every format I can think of, and can create archives in new "7z" format as well as ZIP, GZIP, and TAR.