www.crossbrowsertesting.com Not Free. Interactive. Works great.
<br>www.webpagetest.org Works extremely stable and has a lot of awesome features.
<br>www.browserling.com Runs interactively in the browser, but is slow and buggy.
<br>www.browsershots.org - good but slow
<br>ipinfo.info/netrenderer/ - great but only renders IE
<br>www.browsrcamp.com (Safari only. Dont know which version.)
<br>www.browserpool.de (did not get it to work)
<br>browserlab.adobe.com (did not try it, because it wants flash and an adobe account)