TomK Design

http://www.oldapps.com/category/browsers

This is another browser archive. Not as extensive as the one over at evolt but I did just use it to get older versions of Firefox for a current project.

IETester

Browser Compatibility Check for Internet Explorer Versions from 5.5 to 8

IETester is a free WebBrowser that allows you to have the rendering and javascript engines of IE8, IE7 IE 6 and IE5.5 on Windows 7, Vista and XP, as well as the installed IE in the same process.

This has come in real handy, especially at work where I need to still test sites in IE6. The DOM inspector is very basic and you can not make any edits but it is better than nothing. The DOM inspector in IE Developer Toolbar is much better but it needs to be run from inside IE6.

evolt.org browser archives

evolt.org browser archives can come in handy if you need to test against older versions due to client requirements.

CSSVista

CSSVista is a free Windows application for web developers which lets you edit your CSS code live in both Internet Explorer and Firefox simultaneously. This tool is in a very early version and there are a few quirks. Most importantly, do not edit CSS files which reside in the same directory as the HTML file or your CSS file could get deleted!
It is a very basic tool, but it has come in handy a few times already to help solve some tricky IE problems. I hope they work the kinks out as I could see this as a standard tool in my toolbox.

TopStyle Pro

From Bradbury Software. Written by Nick Bradbury, creator of HomeSite and FeedDemon, TopStyle Pro contains powerful tools for building standards-compliant sites. There is even a free lite version!