As a developer working to design and template sites primarily based upon php drawing from sql I am often reaching back into directories with css files. Often, it proves quite difficult to find that one line or section to make the change to affect real change upon the face of the site.
My #1 tool is Firebug, a plugin for Firefox. http://www.getfirebug.com/
For text editing I prefer notepad++ however when editing pages for Magento I must confess to using Dreamweaver more then I would like.
Both of which are open source.
What does everyone else use?