I have to upload an archive for some Linux, Windows and Mac OS users. I'm using Linux so I'm sure everybody could handle a tar file, Windows users will get some problems, so I think zip is the best way. But how is it on a Mac OS system? Can Mac OS open a zip (or tar) without extra packages/software?
OS X can handle both by default from Finder. The Archive Utility that ships with OS X knows how to extract both formats. Just double click on the file or Cmd-Click (or right click) on the archive and select 'Open' from the list and OS X will unarchive it.
Users also have access to
There's also the free, and most excellent, Unarchiver that they can install that allows for a few more options when dealing with compressed files. It also supports a few more formats. But it's completely unnecessary for extracting gzip-compressed tar archives and zip archives.