DragonFly? BSD is a free BsdUnix operating system that started in 2003 as a fork of FreeBsd 4.8. DragonFly? started over an alternate SMP synchronization strategy than the one FreeBSD 5 ('SMPng') was moving towards - where FreeBSD was splitting the big kernel lock into many small locks, DragonFly? moved many resources to per-CPU pools and used inter-CPU messaging for communication.
DragonFly? has gained a number of cool features over the years - a snapshotting filesystem, HAMMER, process checkpointing, virtual kernels.
Its homepage is located, predictably enough, at http://www.dragonflybsd.org/CategoryOperatingSystemCategoryUnix