Here is a very useful add-on for Firefox users (I am quite sure there are equialent ones for the other browsers). It is called "Firesizer" and it can resize your Firefox window to some exact value for example 1024x768. This is great way to test your webpage if it is behaving well on a smaller display (or overhead projector :P)
Up till now I was using my VMware player to start up Ubuntu virtual machine in order to be able to check my work on a smaller display and I that thought there has to be a better way. Conveniently enough I found this add-on.