Dosboy's Bits and Bytes

In my new pseudo-career as web developer I've been looking for tools to help me out along the way.  And since I haven't done any REAL web dev for a few years, I'm a little behind.  At any rate, what has got to be the absolute most valuable tool I've ever seen is Firebug.

According to their website:

You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.

And you really can!  It doesn't do anything server-side, but all the code that your browser sees is accessible to it for you to change on the fly.  Makes debugging (especially someone else's code) a breeze!  Anyway, if you do even the slightest bit of web dev, download and install it.  I can't do it justice here... you have to try it to believe it.