Fedora

You won't learn a thing about linux running ubuntu, its far too convincing a windows ripoff. (As if in homage, its system requirements are almost equally horrendous, too.) And Redhat linux is closed-source now, Fedora is the open-source fork of the last open thing they had.

For a developer distro, I'd recommend Gentoo -- the only distro I know of where you can do "emerge xorg-x11" and have xorg-x11, headers and all without needing x11-dev, xorg-x11-dev, x11-headers, x11-drivers-headers, x11-kitchensink-headers, and three million others to complete the set before compiling X things will actually work. It also lets you install multiple versions of gcc and switch between them seamlessly. Installing Gentoo the old-fashioned way from the Gentoo Handbook is a crash-course in linux down to the bones.

I've also heard good things about SuSe. It apparently provides a decent developer environment. But again it has the millions-of-unguessable-developer-dependencies-package problem.

Sorry Corona688 but you are incorrect. For example, the complete source code for RHEL in the form of source RPM's is available publicly available ftp://ftp.redhat.com/pub/redhat/linux/enterprise/

Which is what CentOS is based on.

Okay. Thanks a lot -- great answers. And thanks for the recommendation on Gentoo, Corona -- I'm definitely going to look into that one. I'm curious about some things regarding if I wanted to install one or more of these systems on my MacBook (under Snow Leopard -- well, Leopard right now, but I'll be upgrading Snow Leopard before I install a Unix system). I have both Parallels Desktop and VMware Fusion on my MacBook (although neither are being used at the moment -- I don't have any other OS's installed or emulated or anything at the moment). So, I'm curious:

1.) I could, if I wanted to, use Boot Camp to partition the hard drive and then install the OS(s) on those partition(s) -- that would be one way to do it, right?

2.) Instead of actually installing the OS(s) on the hard drive, I could alternatively use emulation, right? In this case, I'm curious -- what would be the effect of using a emulator like VirtualBox or something versus an application like Parallels or Fusion? What's the difference there in terms of its effect on installing an Unix-based OS? Would you prefer one to the other? Will any OS work under VirtualBox, or just certain ones? Similarly, will any OS work with Parallels/Fusion? Also, if I did choose to use Parallels or Fusion instead of VirtualBox, would one be better (between Parallels and Fusion) for Unix-based OS's than the other -- I've always liked Parallels, but I just wanted to see if anyone has any experience in that direction.

3.) So, I could choose which of these two ways I want to do it (partitioning or emulation). Will the OS run better one way than the other? I assume there has to be some downside to emulation otherwise everyone would just use that, right? Is there anything along these lines (partitioning vs. emulation) that I should know? Any input here?

Thanks again everyone -- your responses are very much appreciated.

The questions you directed to me have been answered expertly by the previous posters so I'll refer you to their responses.

As for emulation vs native, native is always better. Having said that, I don't think it will matter much for most situations.

You might want to consider Solaris zones (aka containers). You can create multiple virtual machines using a single image of the Solaris. And you can run Linux in a branded zone. This is probably the most efficient way to run Solaris and Linux environments simultaneously on a single system.

Yes.
VirtualBox, Parallels and Fusion are similar products. VirtualBox being the only one based on open source code and downloadable for free.
I would say any mainstream current x86 OS should run on any of these except Mac OS/X itself which as far as I know can only be emulated by Parallels.
An OS will always run faster without a virtualization extra layer.

Thank a lot, guys -- your replies have been extremely helpful. A few things left that I wanted to wrap up if anyone gets the time:

1.) Like I mentioned, I'm running a MacBook right now, but am most likely going to be buying an expensive PC soon. It was mentioned, however, that it may be a good idea to mess around with installing various Unix OS's on a machine that is strictly used for that purpose, that way protecting the other machines from any side effects of this experimentation, especially since I will be doing a lot of development with the Unix OS's, which can have side effects in itself. So, in place of or in addition to the high-end PC I am thinking about buying, I may look into an inexpensive little notebook for running various Unix OS's. Does anyone have any recommendations for what I should buy if I wanted to do this? I would want it to be cheaper, since it would just be for the sake of experimenting with different Unix OS's and development on those OS's, but I would also want it to be fully functional in that sense (not a P.O.S., in other words) -- so what would be the minimum basic specs I should be looking to buy on a notebook for this purpose that would be as cheap as possible but still allow for sufficient performance for the purposes I've discussed? Also, what type of notebook should I be looking at? Are there any alternatives to the standard PC/Mac decision -- is there anything else out there that I should be checking out? I know desktops can be custom-built from the ground up -- can this be done with notebooks too?

Thank you very much for your time and help!