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