Flickr/Kabils

Greasemonkey is a Firefox add-on that enhances Web browsing experience by allowing users to customize and control the way websites behave, using scripts written in Javascript. The rules are stored by Greasemonkey and come to play whenever the said Web page is opened in Firefox. Greasemonkey is available for Firefox, Flock and Epiphany.

Greasemonkey add-on just manages scripts and would not do anything on its own without any scripts installed. The user scripts have rules that can make a website more usable, integrate functions of two websites into one and lot more.

Once you install Greasemonkey add-on in your Firefox, you need to load some scripts. If you have no knowledge about how to create your own script, you can search for scripts that perform the desired action on User Scripts.

Here is a list some of the popular scripts available for download.

1) Folders4Gmail

Gmail allows you to organize mails using labels but lacks the facility to divide the labels into different categories. Folders4Gmail will enable you to organize your labels in a folder-like hierarchy.
Folders4Gmail

2) Page Scroll Marker

If you lose track of where you were on the page while scrolling, Page Scroll Marker will create translucent lines so that you don't miss it.
Page Scroll Marker

3) Google Account Multi-Login

If you use multiple Google acount, you know that Google won't let you simultaneously log in to multiple accounts through one browser. Google Account Multi-Login adds a drop-down menu to all Google pages, including Gmail, for quickly switching between different accounts. You can add several Google accounts to the drop-down. Of course, it is a bit of compromise with security, revealing your passwords to the script. Else, use two browsers to log in to two Google accounts simultaneously.
Google Account Multi-Login

4) Google Inline MP3 Player

Google Inline MP3 Player adds a [Play] link next to any linked MP3 file, making it possible to play them easily. It quickly inserts Google's player onto the page with the MP3 loaded for streaming. The player can be hidden by clicking [Hide Player].
Google Inline MP3 Player

5) AutoPagerize

As the name suggests, AutoPagerize automatically loads the next page and inserts it in the existing page. So next time you search on Google, you won't have to click on Next link to go to the next page. There is also a Firefox add-on that does the same.
AutoPagerize

6) Gmail Unread Message Count in Favicon

Gmail Unread Message Count in Favicon adds the number of unread messages in Gmail inbox to Gmail favicon in browser tab. The favicon keeps changing colour as the number of unread messages increases. Green icon means no unread message. The Gmail envelope icon turn orange and starts flashing when you recieve a new chat message.
Gmail Unread Message Count in Favicon

7) Tiny URL Decoder

Tiny URLs can be sometimes funny, sometimes disastrous. Many a times these link can be disappointing. Use Tiny URL Decoder to get the original URL behind the shortened URL. You can read more about URL shrinkers and expanders in a previous post.
Tiny URL Decoder

8) Twitter Search results on Google

Twitter is becoming the first source of information which comes from its millions of users spread across the globe. The tweets on the Mumbai terror attack and the aircraft landing in the Hudson river are two examples of Twitter serving as an important source of news and leads. This makes it logical for you to want to search Twitter for any particular information. Twitter Search results on Google will allows you to look at what Twitter has to offer. It shows real-time results from Twitter on Google search pages.
Twitter Search results on Google

9) Twitter Fantastico

Twitter Fantastico is supposed to "autopagerize" Twitter pages, refresh them every single minute and add retweet functionality. It didn't work on my Firefox in Ubuntu. Don't know why? Maybe it loves Windows.
Twitter Fantastico

If you wish to know the structure of a Greasemonkey userscript or want to write one, Drive into Greasemonkey is a free book about programming with Greasemonkey.

Function similar to Greasemonkey are available in other Web browsers as well. Opera has in-built user script functionality which can run Greasemonkey scripts along with Opera scripts. Safari web browser has Greasekit add-on for handling scripts. Open source Konquerer also supports user scripts.