Daring Fireball Projects: Markdown
Markdown is a text-to-HTML conversion tool for web writers. Markdown allows you to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML (or HTML).
aaargh, another of these (I use [very little of] Brad Choate’s textile 2 now, and I mean to take a look at TextPattern as well). Now this . . .