Today I found myself editing a README.markdown file that was to be committed into Github.  I was pretty sure that I had the format right but I wanted to preview the content before committing in order to avoid a bunch of unnecessary commits just to fix my formatting. 

I'm still really a noob when it comes to TextMate but I correctly guessed that it could help me with this problem.  I soon learned there was a bundle that supports Markdown stuff.  After looking around for it a while I realized that I actually already have the bundle installed by default!

Pressing Control-Option-Command-P will open TextMate’s own Preview as HTML window.  Pretty sweet!

UPDATE (07.08.2008)

You can also change the default from Markdown to Textile by clicking options and replacing Markdown.pl with Textile.rb