SmartDown is a single-plugin replacement for the MarkyPants, Markdown, and SmartyPants plugins.
The original Markdown was created by Jon Gruber as a way to write basic HTML, using an easy-to-read, easy-to-write plain text format.
The target audience is web writers, and non-technical web authors (read: clients).
Markdown supports all the text formatting typically required when creating content: headings, paragraphs, links, bold, italics, lists, blockquotes, you get the idea.
The splendid Michel Forin wrote a PHP version of Markdown, and then proceeded to extend it. Markdown Extra is the result, and features even more spicy content goodness, including definition lists, tables, even footnotes.
I may need to lie down, the excitement is too much.
SmartyPants translates ASCII punctation characters into "smart" typographic HTML entities. SmartyPants performs the following transformations:
") into "curly" quote HTML entities.
---) into en- and em-dash entities.
...) into an ellipsis entity.
SmartyPants formatting is applied after Markdown formatting. This means that SmartyPants won't mess with your Markdown code blocks, or otherwise ruin your nicely-formatted content.
Full SmartyPants documentation may be found at Mr. Gruber's online residence.