Used in combination with Firefox, Firebug is an absolute must-have for anyone developing web pages. It shows the styling as derived from the entire CSS cascade, displays on the live page where elements occupy, has a good script debugger, lets you edit styling on-the-fly, and more. There are also lite versions available for IE, Opera, Safari, and Chrome. You can pick up a copy at:

In our opinion, using Firebug is one of the better ways to gain a good understanding of how CSS actually works.