Greasemonkey is a plugin for Firefox browser that lets you assign DHTML scripts to various domains. What's the big deal you ask ? This lets you correct some annoying problems some websites have or even add some nice features to your regular websites.
There are tons of user contributed scripts for doing fun things, like Adding waypoints to google maps., Remove ads from indiatimes.com etc.
The disadvantages of this are 1. this is firefox specific and 2. This works only on the computer that you installed the extension and scripts on. But hey! it's still way cool...
Here is a very nice application of this: Let's say you are browsing amazon.com for some books, how about checking out your local public library's catalogue for the same book and displaying that information right next to amazon book title ? This has been done by Jon Udel (but looks like he has taken down his script) and Bill Stilwell. I managed to tweak his script to search San Diego Public Library. Hurray! Contact me if you are interested. I will polish the script and put it here in a few days anyway...
Some more blog posts on greasemonkey:
Hmmm... [Dive into Greasemonkey](http://diveintogreasemonkey.org/ "Dive into Greasemonkey")
Wow this monkey is getting bigger and bigger.
[Here is](http://www.wired.com/news/technology/0,1282,67527,00.html "Firefox Users Monkey With the Web") a link to a recent wired article on greasemonkey.