bsd

Most of the great innovation that could have happened to FreeBSD during Jordan Hubbard's tenure at Apple were merged into Darwin and never backported back to FreeBSD. Not that FreeBSD is still not a solid system in and of itself (my favorite and current package manager pkgsrc owes its life directly to the idea of the bsd ports tree), but a great deal of innovation happened to Darwin and OS X and not FreeBSD during that time period. Now that Hubbard has left Apple and is currently working on NextBSD (a.k.a FreeBSD trunk with new features) perhaps new life will emerge from the project.

Personally though in recent years I think NetBSD and OpenBSD has superseded FreeBSD in terms of O.S. interest. zfs is cool though.