This is truly a power-user extension. Greasemonkey allows you to write your own code, or draw on hundreds of community scripts (like Book Burro, shown), to modify any Web page. For instance, you could code an overlay for eBay to compare the current price of an item with what you might pay for it at any of a number of online retailers. In the right hands, it can take someone else's idea of how their Web site should work and make it all your own. DOWNLOAD