Apple and the BSD folks have included two of the four editors as part of the base install packages. The first goes by two names and is accessed via the terminal (Applications –> Utilities –> Terminal). They are either pico or nano (note the lowercase). These are almost as basic as one can get and still refer to them as full screen editors. The other editor included by Apple is Textedit. This is entirely accessed by the GUI. It has more features than pico but is slightly larger.
The other two editors can be thought of as program editors. They offer features such as syntax highlighting and bracket matching. Again they are larger because of the greater number of features. The first is Gedit (http://www.gedit.org). To be fair, I’ve only used it twice. It is a port from the Linux world where I prefer to use Kate (which IS available for the Mac if a whole bunch of other software is installed). The other editor is called Text Wrangler and is available from Bare Bones Software (http://www.barebones.com). It is essentially a scaled down version of their commercial product BBEdit.
My preference for non-program documents is Textedit. For programs my preference is Text Wrangler.