(Unless you're a damn fool, you know that there are lots of hobbyists and self-styled rebels out there who are using whatever they happen to find, usually some flavor of Linux. So this post will focus on some other users that you might not know about.)
Linux, Irix, and Solaris, to name a few.
SGI probably has higher retention of outdated/outmoded hardware than Apple does - otherwise, Irix would be totally dead, which you probably knew already. What you might not know is that SGI has a very large installation base among academia and science, because they are so good at modelling and rendering. A lot of them have been repurposed into clusters, but a significant number remain on the desktop as daily work machines.
Linux is also extremely popular in those areas, because the easy access to libraries makes scripting and CLI app development very easy. Getting free software has its perks too, especially for those subsisting on grant money or stipends. It's not surprising to find sociology grad students who use Perl and ImageMagick to reconstitute MatLab data. The openness of Linux has also led to some incredibly awesome single-purpose tools. Since many academic and scientific computing applications have a need for high precision and uncompromising methods, academic toolsmiths all over the world are creating hi-powered tools that will only run in Linux. These tools are so good at their one job that it is worth learning CLI and shell scripting to take advantage of them. These professors, students, and scientists, who are driving the development of Linux, are not quite the "grandma who wants to use email" that many people insist Linux has to market for (we can talk more about that later).
For some reason, enterprise Linux is rarely found at American universities. Many computer resources are therefore going to Solaris, which provides the adaptability of Linux and the reliability of Sun. A few years ago, everything was RedHat. Now, most labs have Sun Workstations, and Linux is confined to professors and grad students who purchased hardware/software with grant money. So the balance between Solaris and Linux is changing.