Some variation of Linux is the de facto standard for Raspberry Pi. However, smaller, lesser known operating systems do exist and some would seem appropriate for such a small device.
Are there any other operating systems that are compatible with the Raspberry Pi?
RISCOS is in the works and there is QT available now. Some bare metal programmers are working on OS's from scratch as well but these are more for fun & research than full blown OS's.
As mentioned earlier, RISCOS is probably in the most advanced state right now. You can also read about making FreeBSD work on RPi here. Some people are working on their own bare metal OSes but they are mostly a hobby projects, here is an example. There are some technical problems with getting different OSes on Rpi, mostly because of the lack of publicly available documentation to some peripherals, especially USB (which is known to have a lot of problems even on Linux).
Haiku, an OS whose design was inspired by BeOS, has an extremely early port available for the Pi. Read here for more details.
You can also run Plan 9 by Bell Labs on the Pi.
http://bendyworks.com/geekville/lab_projects/2012/11/getting-plan-9-running-on-the-raspberry-pi
There is also version 4 of Android being developed and Aros a remake of the Amiga Operating System.
There is also a port of Firefox OS.
It was reported on the official raspberry site too http://www.raspberrypi.org/archives/1787
September 2016 update:
Above link are broken (the Raspberry foundation site one is still functional). At present, september 2016 the Firefox OS porting on raspberry seems stalled.
Firefox Os passed the baton to B2G and the development efforts seems to me somewhat halted.
