It was a little hard finding a self contained message signing page.   It shouldn't be. 
I cobbled a one pager together a sample based off of some js files I found from other projects.

http://mining.tipsternet.com/sign_message.html

If you don't want to install a full wallet, you can take this route.
Read through the page source and construct your own. 
With existing libraries, it can be compacted down to a few lines of javascript code.