Fink is an EXCELLENT package manager-y app available for Mac OS X and Darwin.
The following is the official description from the fink website at fink.sourceforge.net
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"Fink is a project that wants to bring the full world of Unix Open Source software to Darwin and Mac OS X. As a result, we have two main goals. First, to modify existing Open Source software so that it will compile and run on Mac OS X. (This process is called porting.) Second, to make the results available to casual users as a coherent, comfortable distribution that matches what Linux users are used to. (This process is called packaging.) The project offers precompiled binary packages as well as a fully automated build-from-source system.
To achieve these goals, Fink relies on the excellent package management tools produced by the Debian project - dpkg, dselect and apt-get. On top of that, Fink adds its own package manager, named (surprise!) fink. You can view fink as a build engine - it takes package descriptions and produces binary .deb packages from that. In the process, it downloads the original source code from the Internet, patches it as necessary, then goes through the whole process of configuring and building the package. Finally, it wraps the results up in a package archive that is ready to be installed by dpkg.
Since Fink sits on top of Mac OS X, it has a strict policy to avoid interference with the base system. As a result, Fink manages a separate directory tree and provides the infrastructure to make it easy to use. "
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From my personal experience, Fink does what it does very well. I am NOT a *nix-savvy person, though I think I am learning fast, if I do say so myself. Fink made it VERY easy to compile, install and run XWindows, Gnome, and many many other apps on my Mac OS X system, as well as ensure they are all up to date. I highly recommend it to any OS X users wishing to begin leveraging the power of Darwin.