The Unarchiver is a free Mac app that can open almost any archive file. Mac OS is lacking support for many common compressed file formats out of the box. The Unarchiver adds support for many additional formats, including RAR, Tar, and 7z. It’s very simple and lightweight, so for those reasons I’d recommend you look no further if you simply need to open a compressed file on your Mac.
7-Zip is a free and open source archive program for Windows. Windows cannot open many types of archives by default, other than .zip files. If you need to open a RAR, tar, or of course, a .7z file, 7-zip offers a great alternative to paid software such as WinZip and WinRar, and works just as well. This is the most popular option on Windows and I highly recommend it.
HamsterSoft Zip Archiver
HamsterSoft Zip Archiver is another free tool for Windows. It is similar to 7-zip in that it opens many archive formats that Windows can’t, by default. It offers a simpler interface, and some extra features such as uploading to cloud services like Dropbox, Google Drive, or OneDrive, and easily sharing direct links to your files. For those looking for a 7-Zip alternative, check this one out.