I do a lot of my work on my MacBook Pro and as you know not all Browsers run on a Mac. Specifically MSIE (Microsoft Internet Explorer) has not had a new release of their browser for the Mac platform since around version 5. It makes it kind of difficult when doing design changes to a website to make sure that your changes are Cross Browser when you can't install the Latest IE Browser on your Machine. You could set up VirtualBox and download one of the available images from Microsoft setting them up as a virtual machine.
Adobe BrowserLab is a flash application that is able to rend a website exactly how the different browsers render it on different platforms, for example Firefox 3.5 on windows and OS X.