This is a review of Haiku, the open-source version of BeOS from 2001.
Source: IEEE Spectrum May 2012 p. 40.
Haiku uses a thread per window whereas Windows and OS X have only one thread for the whole user interface (what you see on the monitor), so Haiku has less "application not responding" or OS X's "beach ball of death." Haiku installs with 1/30 the hard-drive space of Windows 7 Ultimate and runs on low-RAM systems, yet it makes use of multicore processors. Haiku boots in 15 seconds.
Haiku does not work with some legacy hardware and software, and it is simpler for getting freed up from legacy stuff.
Haiku only has alpha releases, but the volunteers writing it intend to have a beta in late 2012. Drivers and applications are somewhat lacking, but remember that Haiku is free for individuals and is still being developed. Haiku is so far only for Intel x86 processors.
Haiku has a database-like file system, and e-mail attributes are indexed and searched by the OS and the e-mail app.
There is one development group for Haiku and they claim an advantage over Linux flavors, in that Linux apps sometimes behave differently, or don't run, on Ubuntu, Red Hat, etc.