There’s one included in OS X and it’s called TextEdit. It’s in your Applications folder. It’s much more than a plain text editor, though, you can use it as such.
If you want a clean and simple look, you can go to the “Format” menu and select “Make Plain Text,” and the formatting bar and ruler will disappear. But you can also use the “Make Rich Text” command and do extensive formatting, if needed.
Apple’s Knowledge base has a quick tutorial on using TextEdit: Mac 101: TextEdit “TextEdit is a highly versatile word processor, including tools to format and layout your page, edit and stylize text, check spelling, create tables and lists, import graphics, work with HTML, and even add music and movie files.”
And there is also a help file in TextEdit, under the “Help” menu.
The version of TextEdit included with 10.4 will also open the newer XML Microsoft Word document formats. And you can save documents as Word files, too.
TextEdit will also open SimpleText files, if you have some left over from an old Mac that ran OS9.
Another use for a plain text editor is for writing HTML. With the built in Apache Web server on your Mac, you can write Web pages from “scratch” and look at them and learn HTML. Start a plain text document with the usual HTML headers and save it as “index.html” in the “public_html” folder in your home directory. The full path is MyHardDrive/Users/~MyUserName/Sites/public_html/index.html Then, go to your Sharing Preference Pane and turn personal web sharing on, and use the link in the preference pane to go to your page.