Sometimes it's useful to resize a browser to fit into a screen resolution, for example 800x600 or (more commonly these days) 1024x768. This allows you to test using a smaller resolution than you actually have.

You can do this easily in any browser by putting this javascript directly into the url box:

javascript:window.scrollTo(0,0);resizeTo(1024,768);

This tip was found in the preview version of Building iPhone Apps with HTML, CSS, and JavaScript: Making App Store Apps Without Objective-C or Cocoa.

There's also a firefox plugin that allows you to do this, along with a myriad of other things useful for development. See http://chrispederick.com/work/web-developer/ for more details.