FitnesseNotifier is a project mainly written in JAVASCRIPT and PYTHON, it's free.
Google Chrome Extension checking if Tests are running
Fitnesse Notifier:
This extension uses Konstantin Vlasenko's fork of Fitnesse http://vlasenko.org/2010/12/27/fitnesse-tests-in-progress-indication/.
Firstly you should go to Options and set right Fitnesse Server name.