Haiku is an open source operating system currently in development that specifically targets personal computing. Inspired by the Be Operating System, Haiku aims to become a fast, efficient, simple to use, easy to learn and yet very powerful system for computer users of all levels.
This project seems to have been around since around 2006 but I've seen it making the rounds lately. There were parts of BeOS that I loved (such as Workspaces) and I was sad to see disappear entirely. I'm glad there are a few people out there trying to keep it alive.