Xar (archiver)
Xar (short for eXtensible ARchive format) is an open source file archiver and the archiver’s file format. It was created within the OpenDarwin project and is used in Mac OS X 10.5 for software installation routines, as well as browser extensions in Safari 5.0. Xar replaced the use of gzipped pax files.
One development branch of RPM, RPM5, uses xar.
Read more about Xar (archiver): /etc/magic