One of the first thing I have installed on my Mac is Fink - a project that aimed to bring the Open Source software from the Un*x/Linux land to Mac OX X and Darwin. I have installed the latest version (0.5.2 at the point of writing), and basically it gives you the Debian-like package management system. It makes all Debian users feel right at home with commands like apt-get and dpkg. It even has the ever so popular Debian package management system dselect. Fortunately you can also use Fink Commander, which is pretty similar to dselect, i.e. having a flat view of all the packages, but as a Mac OS X Aqua application. However, using apt-get makes everything so easy to use. To install a package, for example gimp, all you need to do is getting onto the terminal and...
$ apt-get install gimp
It should be very familiar to all Debian users. I've used it to install quite a few programs that I used frequently in my Linux land, and they all run flawlessly on Mac OS X. Some of them require X11, while a lot of them are just command line apps. It really makes a Un*x/Linux geek feels right at home ... provided that you have been a Debian user.
As for me, my choice of Linux distribution has been Mandrake Linux for the last couple of years. My experience with Debian has not been good - I used Debian once when it was in 1.2 (back in 1996-1997 I think), and I constantly have stable/unstable package conflicts that result a very unstable system. The same goes the same with Fink - there is the stable branch and unstable branch, and when you start installing stuffs from unstable, welcome to the dependency hell. I've wanted to install PostgreSQL from Fink, but it only exists in the unstable branch. Maybe I will just wait for a bit. I guess a RPM based packaging system will have exactly the same problem. Maybe I should just go back to build everything from scratch :)
Overall, I found using Fink very enjoyable. That's the very reason I switched to a Mac - not just a pretty interface, but underneth there lies the power of Un*x and Open Source software.