There are other providers of this service but I like Adobe® BrowserLab.

It is an online hosted service that lets you test the pages of your web site across a variety of web browsers and operating systems. The service works by taking screen shots of your web pages in different browsers, and then displaying them in the BrowserLab application window.

You can use BrowserLab as a standalone service, or integrated with Dreamweaver CS4. The standalone service lets you test pages that you’ve posted to a server within the context of a web browser. If you use BrowserLab as an integrated service with Dreamweaver, you can test your pages from within Dreamweaver without publishing your pages to a server.

BrowserLab supports the following browsers:

• Firefox 2.0 – Windows XP
• Firefox 3.0 – Windows XP
• Firefox 3.5 – Windows XP
• Chrome 3.0 – Windows XP
• Internet Explorer 6.0 – Windows XP
• Internet Explorer 7.0 – Windows XP
• Internet Explorer 8.0 – Windows XP
• Safari 3.0 – Macintosh OS X
• Safari 4.0 – Macintosh OS X
• Firefox 2.0 – Macintosh OS X
• Firefox 3.0 – Macintosh OS X
• Firefox 3.5 – Macintosh OS X

Adobe® BrowserLab.