Litmus is a web browser testing service that lets you test your web pages against 23 browser versions on 3 platforms as well as testing email on 16 different mail clients. The service offers a very intuitive experience for running tests, viewing the full page screenshots, and verifying compatibility.
They also just released Alkaline, a Mac app that lets you view your tests of Windows clients without going to the web site, which is shown in the screenshot above. This looks like it will be a great service for small teams or individuals. Doing my test with 2 browsers brought back screenshots within a minute or 2.
They offer flexible plans, e.g. a 1 day pass, and subscription plans that are downgraded retain screenshots of previous tests. If your pressed for time, or don't want to fire up that VM, this looks like the service to beat.