Aaron Swartz and John Gruber have unveiled their seekrit project, "Markdown," a system for marking up ASCII to make it readily convertable to styled html text, without sacrificing the readability and expressiveness of the core text. There's already support for Blosxom, BBEdit and MT, and it looks pretty straightforward to implement in other environments.
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)... The overriding design goal for Markdown’s formatting syntax is to make it as readable as possible. The idea is that a Markdown-formatted document should be publishable as-is, as plain text, without looking like it’s been marked up with tags or formatting instructions.Link (via Aaronsw)