7-Zip is open source, meaning it's completely free, even for commercial use. It's only 1MB in size, and can pack and unpack just about any compressed file archive you can throw at it.

With support for Zip, Zipx, RAR, LHA, 7Z, JAR, and WAR formats, WinZip makes it easy to open any compressed file you receive, no matter which platform it was created on.