What to use for a plain text editor?
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.