Part of keeping stylesheets up-to-date involves searching through the styles and removing them which have expired with no further use, previously involving a print out of the stylesheet, highlight pen and a copy of your source code.
A boring but essential part of everyday programming.
This Firefox plug-in is free and will remove all those redundant styles without the boring but necessary job of going through yourself.
Grab it here. 
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Great tool. But if you have large CSS files, deleting all by hand takes too long.
So if you are gonna use it:
Here’s a tool to actually delete the unused css chunks using the results of this plugin:
http://jclaim.sourceforge.net
Start it, in the menu: go to General/Handy Tools/Clean CSS