A "free browser that allows you to have the rendering and javascript engines of IE8 beta 1, IE7, IE6, and IE5.5 on Vista and XP, as well as the installed IE in the same process."

Invaluable for troubleshooting web development in IE. Wish I'd known about this before today!

Free Mac software for taking webpage screenshots. Can capture the entire length of a page (beyond the "fold").

A list of online collaboration tools.