Since Apple started using Intel processors, it's become more feasible than ever to run OS X on non-Apple hardware. Head on over to www.osx86project.org and you'll find a community who are working on doing just that. If you want to join them, be prepared for a fair amount of hacking, though: OS X out of the box only provides drivers for Apple hardware.
The Kalyway install disk fixes this, but you'll probably still need to mess with the BIOS settings. Note that OS X is not free software, so you'll need a licence to run it. On the flip side, you've been able to run Linux on Mac hardware for ages – there's a PowerPC port of Debian, for example – but it's even easier now that Intel chips are in use.
You should be able to boot a live CD without any hacking – so it's easy to give it a go, or to use this once in a while if you just want to run a few Linux apps or check something out. Finally, there's the virtual machine option. Again, if you want to run OS X inside a virtual machine on a Linux box, you'll need to pay for a licence. However, running Linux inside a Mac on a virtual machine is straightforward and painless.
Given that most Linux software is portable to Mac, though, there seems little need to bother with running a distro in a virtual machine. Since OS X is POSIX-compliant, meaning it conforms to the POSIX standards set by the IEEE to define Unix compatibility, software written for Linux or BSD can be easily recompiled to run on a Mac.
From our point of view, this means that we have access to a far bigger stack of software than just the programs natively released for Mac, even though that's an increasingly large quantity of software itself.
There are two major projects that aim to make this process easier: Fink and MacPorts. Both aim to provide OS X ports of pieces of open source software, in particular those that come from the Unix world. They're also fairly easy to use once installed.
However, each one has its issues, and the chances are that you'll be a little further behind the trend than if you were installing the software from a Linux repository. In other words, the Fink stable package will be an older one than the package you'd find on, say, Fedora.
In terms of the differences between the two, Fink has a GUI available and can be used from the Terminal, whereas MacPorts is Terminal-only. In addition, MacPorts is built by Mac employees, which may make it slightly more reliable, and it tends to be a little more up to date. We recommend that you pick one and stick with it, because although they can run side by side, you're more likely to run into problems that way.
Making the most of Macs
As we've seen, there are a lot of similarities between Macs and Linux once you get into the OS a bit, which stems from OS X's BSD heritage. This means that it's straightforward to run both Macs and Linux boxes on your network and to switch regularly between the two. For laptops in particular, the 'it just works' factor of Macs is fairly compelling.
There's also some great software available for Mac that isn't available elsewhere, especially graphical and music-making apps, but using it doesn't mean you have to skimp on using Unix programs. The downside is that Mac hardware is costly and the OS isn't free.
You probably don't want to shift over to Mac altogether, but it's well worth investigating a little, or at the least looking into some of the neat Linux software inspired by Mac design options.