Cross-browser and cross-platform testing is a bane as well as a requirement for web applications. Consider Internet Explorer on Windows. There are two major version 6 & 7, not to mention still used 5.5. You need to either use different machines or multi-boot machines or use hacks to run them simultaneously or virtualization software like VMWare; not of them are really easy to setup and use for common web developer. Then you have Opera & Firefox (we develop in Firefox so it gets tested first in our case). Did I mention testing on Mac with Safari, Firefox and other browsers? Oh, and don't forget Linux too.
