If you use Firefox, and don’t have the monkey’s face staring at you from the status bar, you probably won’t know too much about Greasemonkey.

From wikipedia:

Mozilla Firefox extension that allows users to install scripts that make on-the-fly changes to most HTML-based web pages.

Greasemonkey can be used for adding new functionality to web pages (for example, embedding price comparison in Amazon.com web pages), fixing rendering bugs, combining data from multiple webpages, and numerous other purposes. Well-written Greasemonkey scripts can integrate changes so well that their additions appear to be natural parts of the web page.

I have been using “Facebook Picture Preview” for long enough now to confidently say that without it, my user experience is noticeably stifled. In the words of this guy i know “it’ll change your life” (he used to say it all the time. for anything!).

What does it do? Nothing more than display the full sized original of any picture i hover my mouse over, kind of a super alt tag. So in a gallery of thumbnails, you can see each pic in split seconds rather than having to load each picture individually.

Userscripts is a script “gallery” where you’ll find a load of Greasemonkey scripts for most big sites.

More from el-wiki:

Users have written scripts which:

* Alter Gmail to embed Google Reader into it, thus providing an RSS feed option.
* Show a list of competing retailers’ prices for a book when viewing the book on online retailers.
* Remove advertisements from many sites, including popups and Google text ads.
* Alter the layout of pages, including sizes of elements and screen sizes other than what the author considered.
* Autofill forms.
* Filter specific posters from message board sites.
* Add or remove accessibility features from pages.
* Let a user save an FLV file from popular video streaming sites such as Google Video and YouTube.
* Discover any RSS feeds in the current page, displaying them in an expandable floating panel.

And if you don’t have Firefox, i can only presume you’re not reading this.