I read an interesting article today that was asking if using Linux (or another UNIX variant) was more sustainable than using Windows or a Mac for your home/work computing needs.
The logic was that Linux will usually run quite happily on older hardware, while Windows and OS X tend to want more and more system resources with each new release.
Apart from not needing to upgrade to a new computer, the author was arguing that whatever system it is running on, Linux will use less system resources, less processing power, and therefore less electricity.
In my job I work with Linux every day on remote servers, but my laptop is still running on Windows just because it's what it came with installed and it does everything I need it to, and I had never even contemplated the question of sustainability.