For some time now I've been having problems opening patches that people have been posting on this forum as *.zip.

I've just discovered that these are in fact tar.gz archives.

WinRAR on Windows is capable of extracting tar and gz, but it's not smart enough to detect that the content of the gz is a tar.

So, what I do is extract the gz file, add a .tar extension to the end of the file, then open it again with WinRAR.