A quick collection of links that might help out when learning to use CSS. For many more links see Dan Cederholm's Web Standards Link Bonanza.
Translate CSS selectors into english!
While not really a 'tool' per say, there is so much information about browser bugs and work-arounds here that it is easly one of the most useful websites a developer can bookmark.
Archive of browser bugs, many with fixes/work arounds.
A 3 dimensional view of the CSS box model.
A Great article that explains the CSS Box Model and different positioning schemes.
You will probably see this a lot. It shows you how to handle the differences in box models between IE 5.X/Win and every other browser.
Andy Budd explores margin collapsing.
"For People Who Make Web Sites"
Eric Meyer on print style Sheets
Great article by Mark Newhouse on using CSS to manipulate the display of lists
"Multi-column CSS layouts can run into trouble when one of the columns stops short of its intended length. Here's a simple solution." by Dan Cederholm
Joe Clark takes a look at the accessibility of popular CSS image replacement techniques.
"A demonstration of what can be accomplished visually through CSS-based design."
"What is this? It's a challenge, an experiment, an exploration, a rough map of where we haven't been. It's a search for new ways to approach Web-based design. It's a cry for creativity, and a stab at innovation. It's a playground and a proving ground. It's a rejection of what's practical in favor of what's possible."
Some "real world" web sites that use CSS, not all validate, but all show that standards based design is ready for the main stream: