I came across a product called Alkaline the other day, put out by Litmus. Alkaline is a Mac application that allows you to test a Web site on up to 17 different Windows browsers. It’s much like a Windows-specific version of BrowserShots, but guaranteed to always return quick results and with a few bonus features such as plug-ins that work with common text editors and IDE’s like TextMate or Coda. For more, check out this screencast. You should also check out BrowserShots, if you haven’t already. For the occassional browser test, BrowserShots is fantastic. For more frequent and reliable Windows tests, you may find that Alkaline is worth the money (they have a free version, a day pass, or different subscription rates).
Litmus also makes an application for testing how a newsletter will look in different email clients.