Personally, the Chrome Developer Tools[1] are invaluable for debugging and playing with CSS styles right in the browser. The Pesticide Add-on [1b] is great for debugging CSS because it simply puts boxes around everything. I also use SASS preprocessing [2] to massively improve my development speed and sanity with CSS. The nesting they allow fits right in with a developer's mindset. I'll also throw the Bootstrap Framework [3] in there because it saves tons of time (until you get to a certain complexity and start fighting it) with producing good-looking pages.That said, I still go down long rabbit-holes trying to figure out what's not working properly and feel like workflows could definitely be improved. What tools or processes do you use to debug or develop more efficiently that you couldn't do without?
[1]: https://developer.chrome.com/devtools
[1b]: http://pesticide.io/
[2]: http://sass-lang.com/
[3]: http://getbootstrap.com/