Alternatives to Linux
Alternatives to Linux, excluding the ubiquitous windows
*BSD - These are a series of freely available unix variants, openBSD, netBSD and freeBSD, that are available with an even more liberal license than Linux. They are extremely stable and secure, but currently aren't quite as well supported for the home user as Linux (I expect I'll get lots of comments about this). Evolved from the version of Unix developed at the University of California, Berkley in the 60s and 70s. Worth checking if you are familiar with Linux or other unices, but then if you are familiar with Linux et al why are you reading this?
Mac OS X - This the the current Macintosh OS, very pretty, very user friendly lots of commercial and open source apps available, and using the Fink software it can run any linux/Gnu app as well. It is actually, shock horror!, a unix underneath, running a variant of BSD. Only available with the unspeakably cool looking, but expensive Macintosh computers. Closed source.
RISC OS - originally developed by Acorn Computers fairly soon after the launch of the Archimedes, has from the start been regarded as a very user- friendly system. Today it is as nice to use as ever, although has unfortunately fallen behind in some key areas (a much more up-to-date web browser is high on the wishlist), and the hardware it runs on has fallen behind in the power stakes (whereas at its launch the Archimedes was streets ahead of most competition there). Most tasks that can be done with RISC OS are a joy to do with it.
There are also hoards of other minority OSes out there if you care to look. Old time Amiga fans will be delighted to hear that Workbench 4.0 is about to be released, and there are new 'Amigas' available that run the Motorola PowerPC chips, similar to those that Macintosh use. If you are at all interested in what is out there that isn't windows, or even interested in new developments in windows (double-glazing?), then the website to check is osnews.com