Helping a user run GNU/Linux programs on a proprietary system when one knows that the user has no intention of switching to the use of a free system is generally a bad idea. It is sometimes called "enabling" and is generally considered a waste of one's time.

All the same, there are situations when one is faced with a Mac¹ with strict instructions not to reboot or otherwise "disturb" the existing system.

The news in brief is:

Debian Etch runs with Xorg (vesa driver) under Parallels.
It installs using "mini.iso" for network based install.

Using this one can run all the GNU/Linux programs one needs. It "disturbs" the existing system less than things like "fink" or "MacPorts".

Another alternative is "VirtualBox" from Sun which is free for educational use (and has an "Open Source Edition" too!).

QEMU offers yet another alternative. Since it emulates all the hardware it is significantly slower. However, on current hardware, the speed will be more than adequate if you just want to use standard desktop tools from GNU/Linux.

Pros

1. Applications run in the environment they were designed for.
2. Speed is "native" for running computational code as there
   is no emulation going on for this part.
3. X applications work in full screen mode.
4. Full networking support.

Cons

1. Can only run in 32-bit mode.
2. Currently runs X in "vesa" modes so you don't have
   accelerated graphics.
3. Some keys like F10 and Ctrl+Alt Alt+Enter are trapped by
   MacOS so you may have to remap these keys if your application
   uses them. (For example, Ctrl+T instead of F10 for aptitude).
4. Possible delay in disk access since parallels provides an
   "emulated disk".

Not yet tried

1. Sharing files (Samba/NFS/SSHfs?)