Markdownr.com is a neat little tool I wrote in a couple of hours for myself. I wanted a quick way to preview markdown (if you don't know what markdown is, you should go read about it) since I write all of my blog posts in markdown.
It's a pretty simple site. Just type on the left, and see your rendered HTML on the right. All of the parsing is done with Maruku in a Rails Metal, so it's real fast.
I wanted to be a good citizen, and give it all away, so you can get all of my code on GitHub. Feel free to fork and improve.