Current Mac OSX versions usually unzip and zip seamlessly through the OS (using built-in Archive Utility). Double-clicking should do the trick.
I also use StuffIt Deluxe, which is a for-pay program, but they have a simple app for decompressing all kinds of files (stuffed, zipped, RAR, etc), for free. Go to Aladdin Systems
for the free Expander for Macintosh. For Windows user, go here