From http://www.haiku-os.org/about :
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.
What I like the most is the logo.
The web site is very pleasant and has a lot of documentation , screenshot, ....