I feel a worthy, yet absent, *BSD would also be DragonFlyBSD.
Exactly what the difference is between DragonFlyBSD and other BSDs I will leave for someone else to summarise in a more definitive way, however my take is that it is all-round more "modern".
Dragonfly BSD is a "logical" continuation of FreeBSD 4.x. FreeBSD changed how it handles SMP in 5.x, while Dragonfly seems to be geared towards kernel level clustering.
Exactly what the difference is between DragonFlyBSD and other BSDs I will leave for someone else to summarise in a more definitive way, however my take is that it is all-round more "modern".