ReText

ReText is a cross-platform WYSIWYG-ish Markdown editor.

Toggle editor and preview layout with Ctrl+E.\
Press Ctrl+L to split the window for a live preview.

I use it to copy Pivotal Tracker story descriptions over, switch to the preview and have it as a small always-on-top window in a corner of one of my screens.

On Ubuntu, you can use a PPA to install:

sudo add-apt-repository ppa:mitya57/ppa
sudo apt-get update
sudo apt-get install retext