I've already run across a number of developers who aren't aware of this, and it's too good to pass up, so here goes.
Internet Explorer 8 comes with built-in developer tools. That simple, that nice. Wherever IE8 is installed, you can select Tools | Developer Tools, or just press F12. There a bunch of useful things you can do with the tool, and I'll point you to the MSDN page on Discovering Internet Explorer Developer Tools for details.
Another thing which is a tad tricky if you don't know to look for it is how to dock the tool window into the browser. The tools come up as a standalone top-level window the first time, which is very handy if you have multimon, because you can just move it to another monitor and maximize it (my prefered setup).
But when working on my laptop, the most convenient layout is to dock it in the browser. There is an icon with an arrow on the top-right corner of the window, right above the hourglass on the "Search HTML" edit box, or right beneath the window close button. Click there, and your window will be docked inside IE, so you can see both content and tools at the same time.