Firebug
I'd like to rave briefly about a tool called Firebug. It is absolutely astonishing, and if you are doing complicated web development, almost invaluable.
Lots of the new stuff I'm doing on CanalplanAC involves Javascript interacting with the document. Firebug lets you see the structure of the document as it is at this moment, and when it changes, what you see changes.
That, alone, would be wonderful. But what Firebug gives you is a live editor for the page - not just the HTML but the CSS as well. So you can see what happens when you change the background colour, or padding, or floatiness of a particular element.
The time saved by tweaking the page, rather than editing and reloading is astonishing.
If you are doing almost anything on web pages, you'll find it worth giving a try.
This is an entirely unsolicited testimonial.