John Gruber and Aaron Swartz's Markdown is a plain and open tool for making blogging and other webby things easy and fun. It lets you represent the most common HTML tags that would be used in day-to-day web writing as intuitive punctuation. For example, instead of linking to Markdown by writing
<a href="http://daringfireball.net/ projects/markdown/">Markdown</a>
I think the simplest way to use it on a Mac is to copy Markdown text to the clipboard, then run the following script:
A marked-down copy of your text will then be on the clipboard. I pop up Launchbar to call the script in a jiffy, and using a clipboard with history helps manage the whole mess.