DMGFirebug
Firebug is a JavaScript, CSS, and HTML debugger that runs as a Firefox plugin. Ned Batchelder recommended it, and he's a smart guy, so I installed it on my work and home development machines to see what it's about.
I've only used it twice, and only for a total of about 45 seconds. But it has already saved me hours of work and frustration. This means two things: it's incredibly useful, and it has zero learning curve. I'm floored.
If you're doing any kind of Web development work, Firebug ought to be in your toolbox.