Compression programs are nothing special by themselves. But 7Zip is a free file compression program. It is based on an OpenSource algorithm and as a program itself is also OpenSource. At the same time it has one of the best compression rates. In my opinion best conditions, as format of choice… You can get it here or alternatively locally from here (32 bit) or here (64 bit).
Of course, you can also use WinRAR, but this way you save stupid nag screens or license plays and still have the full range of functions.