If you ever find yourself doing layouts on the web, you owe it to yourself (and your clients) to take the YAHOO! CSS Foundation for a spin. There are other CSS frameworks out there (I really prefer blueprint‘s naming conventions for example, and am looking forward to seeing future updates), but not all are as robust and extensively tested as the YUI CSS components.
This video presentation by Nate Koechley, the author of the library, is a great way to start. Not only does he provide an overview of the tools, but he does a great job of describing some core CSS concepts. I especially enjoyed the color coded illustrations of inheritance and selectivity (about four minutes in).