What to use for a plain text editor?

I'm writing instructions for a project and at some point, for a certain condition one needs to modify a simple plain ascii text file.

For windows I can tell them to fire up notepad. I thought on the mac I could use TextEdit but the TextEdit version I'm using defaults to write rtf, which introduces all rtf symbols in the file which is highly undesirable.

Using emacs or vi from the terminal is not something I want to burden the user with.

What are alternatives?

I would recommend using TextEdit since it is free and the default text editor on OS X.

You only need to learn a key shortcut (Shift-Command-T) to convert the current file to plain text.

Of course other GUI text editors exist on Mac platform but they need to be installed from the App Store or from Internet (see other answers for a selection of the most popular ones).

There is even a setting in TextEdit to let all new documents start as plain text going forward:

Again, this setting is for new documents, so if a document is already open, ⇧⌘T (Format > Make Plain Text) instantly converts a rich text document to plain text.

My recommendation would be the awesome free app TextWrangler from Bare Bones Software

I also recommend TextEdit because it'll be there, no installation needed.

If you only need to modify an existing file, then giving it an .txt extension ensures that it opens in TextEdit in plain text mode, no surprises.