The Evolt browser archive
Most of the time when I find a useful website I post it on my del.icio.us account and leave it at that. This site was so good though I figure its worth its own post.
The Evolt Browser Archive on the Evolt.org web designer network site is simply amazing; Adrian Roselli and a slew of contributors have created the most comprehensive online archive of older web browsers of any site I've ever seen. The potential for site designers (and nostalgic 90s web surfers) is huge; from this one site you could download virtually every older browser and test your designs and site functions in them.
Aside from the regular Internet Explorers and Firefoxes, they also have such gems as iBrowse for the Commodore Amiga, classic NetPositive for BeOS, WorldWideWeb by Tim Berners-Lee and the long since dead NeoPlanet, the browser I used for a while when I was in primary school! They're not limited to graphical browsers either; Links and Lynx both have a huge number of versions available.
My only gripe would be that their emphasis seems to be on proprietary and older browsers; Konqueror, Epiphany and my lovable speedy Dillo (that didn't sound right) aren't archived at all. The latest versions of these browsers are easily installed with your Unix-like system's package manager though I guess.
I pass this site on to you simply because it has made my life much easier, and if you're a designer I'm sure it would make your life easier too. Now if you'd excuse me, I'm off to install Netscape Navigator Gold 2.0 on my Windows 95 virtual machine.