unix
unix has been around for eons (in computer terms) and there is a wealth of info to be learned from the unix comunity. one of the main benefits is that just about any unix source code can be compiled for the mac. but here are my 4 suggestions:
1) go to sourceforge.net and get a basic unix run down from
http://homepage.mac.com/rgriff/files/TerminalBasics.pdf
2)download and install the mac osx developer tools. this will give you a basic c compiler.
3) download a little app called fink. fink is a a debian based file organization/download/install/uninstall program for people that really don't want to worry about ./configure. It's a great app and will give you acces to many many unix apps and scripts right from the terminal.
4) Install xfree86. xfree is a window manager that most unix apps will use. currently, source has to be written for aqua in order for it to work. So, xfree runs in tandem with aqua (or can replace it if you choose) and will let you run everything from gimp to scorched earth (one of the first rpg's) to axyftp (amazing ftp app).
that should get you started.
oh if you want a more "desktop os" feel to xfree you might want to consider installing gnome. Gnome is a little slow on an older mac and sometime can be buggy, but for someone used to a desktop, its second to none