As well as the choice of case there are a number of options for operating system for the Raspberry Pi.
Without getting too technical, the Pi has an ARM based processor so will only run an ARM compatible operating system. Both Windows (up to windows 7) and Mac OS X run on x86 based processors so are not compatible with the Pi. The main operating system that the Pi will run would be Linux.
Linux comes in a number of different distributions or flavours, there are over 600 in total. Each distribution is based on the same Linux Kernel which is the main component of the operating system that sits between the hardware and the applications. The kernel is then tweaked and changed depending how the operating system is going to be used and which bits are needed, for example a version being used as a firewall would need different functionality to one being used as a desktop computer. Different companies and user groups work on the development of each distribution and its applications.
A number of Linux distributions and other operating systems that have been developed to run on the Pi, here's a few of the more well known and popular ones.
Raspbian a Debian based operating system and is optimised for the Pi. It's the suggested distribution and so is a popular choice. There are a number of suppliers who sell SD cards with Debian pre-installed on them. It includes a Mozilla based web browser (Iceweasal), graphics and office suite (Calligra suite) and various programming languages and tools such as Python and GCC. I have bought one of the pre-installed SD cards with my Pi so will use this for a while.
Other Linux Distributions
A number of other Linux distributions are reported to have been ported to the Pi, these include Gentoo, Puppy and Fedora. These will offer basically the same user experience as Raspbian so I probably won't look at these too much for the moment.
Google Chromium OS
Another operating system that is being ported to work on the Pi is the Google Chromium operating system. This is the open source version of Google Chrome, which is a stateless operating system that works with web applications. A good video to explain what this actually means is the What is Google Chrome OS video on YouTube. The Raspberry Pi version of this operating system is in the first stages of being ported and so there isn't a fully stable version yet. Google Chrome OS looks like an interesting concept so I will probably try this at some point once one has been developed into a stable version.
Media Centre OS's
There are a number of operating systems based on XBMC which is specifically designed to work as a media centre, these include Raspbmc and OpenELEC. It works in a similar way to Windows Media Center and Front Row on Mac OS, and supports a variety of audio, video and image formats. It also supports remote controls, these can either be standard tv remote controls which connect using a sensor plugged into a USB port on the computer, or they can be specific smartphone apps that connect to the computer via Bluetooth. Some versions have plug-ins for web services like YouTube and Spotify. I'm interested in trying one of these XBMC based operating systems as I would like to try the Pi as a media pc connected to the TV.