I'm surprised how many people I talk to who aren't aware of Adobe Browser Lab.

You can view the site here: http://browserlab.adobe.com/

It currently lets you type in a URL and compare that in different browsers, including FireFox 2.0, IE 6.0, Chrome, Safari and all the modern browsers. The site works very well and is relatively fast. Very useful if you don't have access to an older browser but need to check a site layout.

They have an especially nice feature, the "Onion Skin View" that lets you overlay one browser over another and shift between them to see the differences.

The only downside is that this is only for display purposes and none of the functionality (i.e. JavaScript) works. It would be awesome to be able to test JavaScript in the different browsers.

Maybe in Version 2.0?