Haiku is an open-source implementation of BeOS, which was the first OS designed for symmetric multiprocessing and preemptive multitasking. Haiku is named for the peculiar error messages from the default browser in BeOS.
The Haiku project is the most feature-complete original OS targeted toward general users that is not related to the Windows, Linux, or OS X / BSD family. While still in Alpha territory, it's generally stable enough to browse the web, listen to music, and do other basic tasks.
Killer feature: Super light-weight, and it doesn't require much in the way of system resources, making it a good choice for migrating on older systems.
Image: Haiku, Inc.