I’ve written a text-to-HTML formatting tool called Markdown, which is now available for download. Markdown allows web writers to compose text using a simple, readable, plain text formatting syntax; Markdown takes care of translating it to valid XHTML (or, if you prefer, HTML).
So, yes, before another person asks, it is sort of like Textile. Except that it’s different.
Markdown works as a plug-in for Movable Type and Blosxom weblogs, and also as a standalone Perl script (which means you can use it as a text filter in BBEdit), and is licensed under the GPL.
If you’re not interested, I’ll try to change your mind in a bit.