On having layout — the concept of hasLayout in IE/Win
Deep inspection of hasLayout, the cause of most 'IE bugs'. Last updated 2008.
When Can I Use
"Compatibility tables for features in HTML5, CSS3, SVG and other upcoming web technologies."
IETester
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."
Microsoft: Internet Explorer Developer Toolbar
Invaluable for troubleshooting web development in IE. Wish I'd known about this before today!
Jeremy Smith's Blog: Success! IE 6, 7, and 8 Running on Same Machine (Plus 5.5 and 5.01)
ies4osx
Run IE5, 5.5, 6, and 7 in OS X under X11 emulation. It actually works perfectly and installs in minutes with a few clicks.
Conditional Comments for IE