If you need to test in multiple browsers, you really need to run multiple operating systems (screenshot services are great, but they don’t give you the live interaction testing you really need). There are a number of great options for doing this. Since I work primarily on a Mac, I run Parallels. I have Windows XP and Ubuntu Linux running in Parallels. This makes it really easy to switch between different systems and browsers for testing.

Of course you can also check out VMware Fusion, Wine, or VirtualBox. If you know of other solutions, post those in the comments. Some of these companies also offer other software that allows you to run multiple operating systems on a PC system, instead of a Mac.