I've only been working with Joomla for a few months but I've found a couple of tools that have really helped in customizing the excellent extensions I've chosen to include in my website (under construction at www.twinbridgeschamber.com).
(1) Firebug - Great for figuring out what CSS classes and files are being applied to a page. http://getfirebug.com/ (2) Windows Grep - Helps you find particular pieces of code in an extension so you can edit them. http://www.wingrep.com/
I didn't discover these tools until fairly recently and they would have saved me hours in customizing extension. Joomla vets probably have been using Firebug and Windows Grep as a matter of course (or maybe can suggest even better tools?).
Both tools are open source (free) but contributions are welcome to help support further development.