Homepage: https://github.com/netblue30/firejail and http://firejail.wordpress.com/
Why should this be included in the repository? Solus' repo has got a grand total of 0 sandboxes and firejail is quite good at what it does and mitigating security holes. More on that here. Its ease of use is quite something too.
Is it Open Source (yes/no): Yes, GPLv2.
Link to source tarball/zip file: https://github.com/netblue30/firejail/archive/0.9.44.tar.gz
Note: certain profiles (eg: firefox) with --private-dev need to be tweaked (to remove the option), as this option will mount a tmpfs directory on top of /dev/shm and, due to this unresolved pulseaudio bug delete pulseaudio's shm files.