The BSD Projects
FreeBSD: The flagship BSD project with solid academic roots.
DragonFly BSD: Originally a fork of FreeBSD, DragonFly BSD is now a superb BSD with many unique features.
NetBSD: Of the BSD projects, by far the most portable.
OpenBSD: OpenBSD focuses on proactive security and code correctness.
And of course:
TrueOS: Now specifically focused on servers. Sponsored by iXsystems.
Project Trident: Formerly PC-BSD, this desktop version is quite versatile. Also sponsored by iXsystems.
Also of note: