Most Raspberry Pi's run some form of Linux (Raspbian, Ubuntu, etc) but you can also run other Unix variants on the Raspberry Pi. Before I started using Linux in the mid 90's I used FreeBSD - an extremely robust operating system. The good news is that Free BSD for Raspberry Pi is now available and I'll talk you through how to install it.
First you'll need to download the images for Raspberry Pi 2 Model B FreeBSD from here or download them from your command line as follows:
There are also other images available for earlier versions of the Raspberry Pi. Just point your browser here to browse the latest ARM images.
Next you'll need to decompress the disk image. You can do this by entering:
xz --decompress FreeBSD-11.0-CURRENT-arm-armv6-RPI2-20151016-r289420.img.xz
Finally, to create your Raspberry Pi Free BSD image, insert a SD card into your card reader and enter the following at your terminal prompt:
dd if=FreeBSD-11.0-CURRENT-arm-armv6-RPI2-20151016-r289420.img of=/dev/sdc bs=1M
remembering to replace /dev/sdc with the name of your actual device.
Now insert your finalised SD card into your Raspberry Pi and turn on the power to boot from it. When you get to the log in prompt. the default passwords are are freebsd/freebsd and root/root.
You should now have a Raspberry Pi working with FreeBSD.