I don't want to spend money, but I don't want to get spy/adware. So, does anyone know a good free program?
Get 7-Zip. Open source, handles pretty much every format. Assuming Windows that is. Mind you Windows has had built-in unzip capabilities since XP.
WinRAR works for ZIP folders and RAR folders as well as oodles of others, but ZIP and RAR are the only ones used I've seen in my limited experience.
OSX also has a built-in program to unzip files so I'm curious as to how the problem came about. It's usually only when faced with a RAR I end up downloading something else.