On many (older) Macs, OS X cannot be installed at all, so Linux is a great way to use UNIX on those Macs. Actually you can install it on a couple of "unsupported" Macs with the freeware XPostFacto for example, although I don't think that it makes much sense considering the way OS X performs on my old G3/266 :-(
XPostFacto: http://www.versiontracker.com/moreinfo.fcgi?id=11168&db=mac
There are other UNIX alternatives -- *BSD, for example -- but Linux is the most popular and, I believe, easiest to use of those possibilities. Especially easiest to install :-)
- Isn't OS X better?
Define "better". Better in what way? If you want your Mac to do server tasks, then Linux definitely has the advantage here -- better performance, better compatibility with other UNIX platforms (especially other Linux types), proven stability and so on. For a lot of server jobs OS X should work nicely as well (especially as a Workgroup Server. I'm not sure how it performs under really heavy load). The most popular servers have been ported to OS X and there is a wide range of "UNIX software" available through Fink (Fink uses Debian tools like dpkg and apt-get). Most of the time OS X does require more resources than Linux
<advertisement gap> Why don't you try the nice SuSE distribution? Well maybe better wait till 8.0PPC is released... though. The login panel alone eats quite a couple of Megs of your RAM (although by changing /etc/ttys you can probably boot into a console). And as far as stability is concerned: seems to be ok so far (didn't heave a single system crash since 10.0).
In many ways, for example, software development on Linux is more compelling an option than on OS X. As you mentioned before: probably depends on what you are developing :-)
Linux also generally runs faster (and feels *much* faster) on the same hardware than OS X. I see you don't use KDE ;-)
After all, Linux runs quite well even on very old hardware (like an old 486 PC or 68K Mac), so naturally it will benefit greatly from the greater firepower of a newer Mac. Also, Mac OS X's graphic layer (Quartz/Aqua) is very processor-intensive and the whole system feels sluggish as a result, especially in comparison to Linux. True, true, true
This may change once OS X 10.2 and QuartzExtreme come out, but I personally am not hopeful in spite of breathless press reports. The problem is that older graphic cards won't be supported at all and 32 Mbytes (on the card) are required for optimum performance as they put it.
As a concrete example, I recently was talking about DivX movies with colleagues. A colleague saw another colleague using an iBook with Linux, and they were using xine to watch DivX movies and DVDs, at very high quality -- much better than under OS X with QuickTime and the DVD Player *on the same computer*, indeed the colleague remarked that he'd never seen DivX movies play so well before. DivX performace is a bit of an unfair example considering that there is still only an alpha version of the codec available for OS X. Anyhow you can still use Xine under OS X as well if you like ;-) (did I already say Fink?)
Gruss Marc