Today, I got a serious CSS bug with IE 6 (still don't know what's going on, maybe the HTML code is crappy...); anyway, just to say that I only have Internet Explorer 7 / Opera and Firefox and couldn't test/debug with Internet Explorer 6 (because there is no trouble with ie7).
I found that good stuff: Internet Explorer 6 Standalone version! Definitely useful...

That makes me think of a tool I can dream of... a kind of meta-browser that support lots of old/current standalone engines. Imagine with your firefox interface be able to load Internet Explorer5 or 6, Opera 8, Netscape 4, Lynx... that would be awesome for web developers/designers...
There is a firefox extension IE Tab which allows you to have the current IE engine in FF, but still, I need the old versions!