cssgoogle-chrome
I used to rely on Google Chrome Inspect Element → Resources Tab → Stylesheets feature where I would double click anywhere in the css source and I could edit it with live rendering in the page.
It doesn't seem to work anymore and that makes me sad.
(I can do the same thing from the 'Sources' tab, but this requires more clicks)
I run Chrome v 21.0.1180.49 beta on OS X 10.6.8
PS. I found somebody else that has the same issue.
Inspect Element is Chrome's version of Firebug. It's not quite as full-featured, but it is still a very useful tool.
Make sure that the Resource tab is enabled - if it isn't, you won't be getting any load times. Click the big "Enable resource tracking" button.
(Update 2014) Since support of user stylesheets was recently removed from Google Chrome, the only option for this moment is to use extensions (like Stylus *1), but these will behave differently (see below).
The most relevant request for re-introducing true user stylesheets in Google Chrome is Issue 347016: Support user stylesheets.
Per specification, "true user stylesheet" rule has lower origin specificity in cascade than author rule, but !important user stylesheet rules have higher origin specificity than !important author rule, regardless of its selector specificity.
Extensions mimicking user style-sheets in Chrome simply injects (hopefully) last style element into page, what have some consequences:
!important rule has higher specificity than one you want to override*1 Original extension, Stylish, is currently (2017) in state of erratic development by a new mainetainer, so I'd advise to avoid it and use alternative like aforementioned Stylus.
(Following original answer is now obsolete.)
Afaik it is not possible to tweak the UA css itself, but you can make a global userstyle: start Chrome once with --enable-user-stylesheet. This will create <user-data-dir>/<profile>/User StyleSheet/Custom.css which you can use (changes are propagated immediately).
http://code.google.com/p/chromium/issues/detail?id=2393
Best Answer
It seems that for Chrome 21, live css editing has been moved to the 'Sources' tab. Huge relief that the functionality is still there!
https://groups.google.com/a/chromium.org/forum/?fromgroups#!topic/chromium-bugs/mvGOga4O4HM