The Unarchiver is a macOS app, but the underlying unarchiving code can run on other OSes, too. There is not yet any GUI app for any other OS, but there are two command-line tools, unar and lsar, which can be used on macOS, Windows and Linux.
Pre-compiled versions for macOS are available on the Google Code download page, or here:
For older macOS versions, use the legacy version:
The Windows versions are currently cross-compiled with Cocotron on macOS. You find binary versions here:
The Windows version is currently slightly outdated. An update will follow.
Ubuntu 12.04 packages unar and lsar, under the package name "unar". You can install them from the Software Center, or using the command line:
sudo apt-get install unar
Some other distros also package unar, but since I do not use them, I am not sure how to install them. Debian probably handles it similarly to Ubuntu above, and I think Gentoo also has some way to install it. If anyone wants to supply me with instructions for those, I will add them here.
You can also download the source code and compile the tools by hand. To do this, you will need to find and install the required third-party libraries that the XADMaster library depends on (GNUstep, zlib, libbzip2, OpenSSL, and ICU). How to do that depends on which distribution you are using. On Ubuntu, the following should work:
sudo apt-get install build-essential libgnustep-base-dev libz-dev libbz2-dev libssl-dev libicu-dev
make -f Makefile.linux