xz is a compression format.
It's in ports:
http://www.freshports.org/archivers/xz/
From FreeBSD 8 and onwards it seems to be in the base system.
xz homepage with versions for different OS's:
http://tukaani.org/xz/
As for 7-Zip, xz seems to be supported, at least in versions "7-Zip and LZMA SDK versions 9.00 alpha and later" (From xz page).