Ok, this is a great idea:
Rendr is a live CSS and HTML rendering tool. It displays what the page would look like as you type, making it great for rapid testing of page designs. Loads in Firefox only. (via Digg)
This is a huge help to me, since I have only a passing familiarity with CSS and my HTML skills are largely confined to cutting and pasting. This is going to make learning these about 10 times faster.