At the last Free software Melbourne (Australia) meetup - we had a great show
and tell session for a load of great programs.
The ones we covered where as follows.
Screen ruler - You can place an on ruler on you screen so that you can easily
measure elements.
Red shift - Allows the screen to remove blue tones late in the night so that
you can sleep easier.
Pandocs - Turns a bunch of mark down comments into a HTML based presentation.
Plus, An incredibly cool yet to be named/completed plugin for Firefox based
browsers that allows the audio to be pushed out to jacked.
I would recommend them all!