[Greasemonkey][gr] is currently not only just the hottest Firefox extension (or plugin) in town, it is also the most disruptive of all browser plugins. It beats me how nobody had thought of such a browser add-on before.
Greasemonkey allows the user to change their experience on _any_ website, on their own, without the help of the website developers themselves. It does this by modifying the website behavior using custom javascript that is added on top of the browser pages.
For example, I am currently using the [Degradr][deg] Greasemonkey script to replace the annoying flash images in the Flickr photo pages with static images. Before this I was literally begging that the Flickr staff would add this feature somtime soon. Now with Greasemonkey, _I_ have this power to change my experience on my own, _right now_. Heck, there is even a [script][toiscript] for cleaning up clutter in the [Time of India][toi] site.
This is disruptive because website owners now no longer have complete control over how users use their website. This is disruptive because it brings across a new paradigm into play – that website content is a mere resource that users may use in whatever manner possible. Let me repeat, this is disruptive because website owners no longer have _control_.
And ofcourse, not everybody likes change. Take a look at [this post by nivi][nivi], and you can see why. You would now be able to lookup an Amazon page, and have an automatic popup giving you the prices of the product you are viewing in other shops.
Then take a look at the exhaustive and growing [list of Greasemonkey scripts][list] which can change the way you look at websites for ever.
[gr]: http://greasemonkey.mozdev.org/
[deg]: http://www.randomchaos.com/software/firefox/greasemonkey/degradr/
[nivi]: http://www.nivi.com/blog/article/greasemonkey-and-business-models/
[list]: http://dunck.us/collab/GreaseMonkeyUserScripts
[toiscript]: http://overstimulate.com/userscripts/indiantimes.user.js
[toi]: http://timesofindia.indiatimes.com/
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