One of the most useful plugins I have ever found is one called Firebug, Let me quickly qualify this by saying this is a Firefox plugin not a WordPress plugin
What Does FireBug Do?
It gives you a visual representation of the CSS that is used to code your site. It shows you which css files particular components use.
In other words it is a brilliant tool to help you write and code up WordPress themes.
It’s not the easiest thing to explain so I have recorded a quick screen cast video of me using Firebug where all will (I hope) become clear.
Download Firebug
You can download and install Firebug from this location http://getfirebug.com/
What I Use It For
I use it all the time to edit CSS on clients sites, for example they may come to me and say I want to increase the font size on my sidebar. Using firebug I would scan through the css, find the item to change, perhaps test the change dynamically in Firebug and then edit the CSS file directly.
Screen Cast Video
[media id=15 width=512 height=384]
Wrap Up
It’s a brilliant tool I use daily on my client’s sites give it a go.