For unarchiving, grab The Unarchiver and maybe Stuffit Expander for the odd once-in-a-blue-moon file that the Unarchiver can't handle.
For making archives, just use the Finder's built-in zip functionality. If you want to make your archives free of Mac-specific stuff to improve sharing with Windows users, CleanArchiver is nice and can compress to a number of formats.
All of the above options are totally free.