Just the other day I was talking with a friend about how tedious in-browser testing really is. Anyways, by looking at some of the latest releases it looks like we are slowly getting somewhere.
Microsoft has introduced at MIX09 the Web SuperPreview for Windows Internet Explorer (nb: runs only on Windows)
Expression Web SuperPreview for Internet Explorer is a visual debugging tool that makes it easier to migrate your Web sites from Internet Explorer 6 to Internet Explorer 7 or 8.
There is also the Xenocode Browser Sandbox:(nb: runs only on Windows)
The Xenocode Browser Sandbox allows all popular Windows browsers to be run simultaneously, directly from the web. Web designers, system administrators, and other users can now evaluate Internet Explorer 8, 7, and 6, Mozilla Firefox, Apple Safari, Opera, and Google Chrome directly from the web in a risk-free manner. Unlike traditional software applications, Xenocode WebApps do not require any software to be installed and allow multiple application versions to run side-by-side on any version of Windows.