Unfortunately, I just have a few different virtual machines with Windows XP (1 unpatched (IE6) and 1 patched (IE7)). I run Windows 7 with IE9 (if it works in IE7 and IE9, its a safe bet that it works in IE8). I also have Firefox and Chrome installed. If it works in Chrome, it (should) work in Safari since they both use Webkit. Lastly, I have 1 older version of firefox installed on one of my VMs (Firefox 3) because that is what my job still uses.

Anyway, it sucks. That's one of the reasons why being a web developer is grueling work, especially when working with older versions of IE. At this point, I pretty much just do a check to see if they are using a browser that has been released in the last 5 years. If they aren't, I display a warning (not an error) and say that the website might not function properly.

This can be a useful website though, if you are purely looking for visual compatibility: http://browsershots.org/

Hope this helps.