I write my Readme in marddown, and often it gets messed up and then I have to commit & push until its finally right, so I wrote a small script to get a preview (using githubs markdown library)!
Usage
markdown README.markdown ... chrome pops up with the html preview
Install
gem install redcarpet
Put this into e.g. ~/bin/markdown and do a chmod +x on it
#!/usr/bin/env ruby
raise "gime a file!!" unless ARGV[0]
exec "redcarpet #{ARGV[0]} > /tmp/markdown.html && chromium-browser /tmp/markdown.html"
Alternatively: Interactive Live Preview<
Michael Grosser, the Blog 
Hey cool, thanks for sharing.
Ubuntu users, remember to do a `apt-get install ruby1.8-dev` before installing a new gem.
Then you can do e.g. `sudo gem install rdiscount -s http://gemcutter.org`
This is great, thanks a lot!