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TextEdit is the default if you exclude the ones that are available with Terminal. 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).

You can change the format for new documents from TextEdit's preferences:

Alternatively, if a document is already open, ⇧⌘T (Format > Make Plain Text) converts a rich text document to plain text.

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.

You can change the format for new documents from TextEdit's preferences:

⇧⌘T (Format > Make Plain Text) converts a rich text document to plain text.

TextEdit is the default if you exclude the ones that are available with Terminal. 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).

You can change the format for new documents from TextEdit's preferences:

Alternatively, if a document is already open, ⇧⌘T (Format > Make Plain Text) converts a rich text document to plain text.

TextEdit is the default if you exclude the ones that are available with Terminal.

Of course other GUI text editors exist on Mac platform but they need to be installed from the App Store or from Internet.

You can write instructions to save as plain text in TextEdit.

EDIT: Here are the instructions:

In the Preferences pane, in New document, set the Format to Plain text. Then, whenever you save a document, you can choose between UTF-16, UTF-8, ASCII and other text encodings.

Alternatively, if a document is already open in RTF, you can switch to plain text with the Format Menu, then Make Plain Text.

You can change the format for new documents from TextEdit's preferences:

⇧⌘T (Format > Make Plain Text) converts a rich text document to plain text.