Jeff Anderson offers a succinct definition: "A mentor text is any text that can teach a writer about any aspect of writer's craft, from sentence structure to quotation marks to 'show don't tell'" (Mechanically Inclined 16).
If you choose to teach writing or grammar apart from workbooks, as I do, it is helpful to have a collection of texts to use as models. Once kids have studied the examples, they can imitate the author, using the same techniques, developing their awareness of options beyond what they currently use.
What I have in each of the categories now is only a beginning. I will continue to build this section as I go. If you want to help me, send me your selection, including the author, title, page number, and the category it best illustrates.