Hundreds of Greasemonkey scripts to enhance, beautify and change Gmail are available on Userscript.org, the website that lists every Greasemonkey script available. Firefox users can use the Greasemonkey extension to add Greasemonkey support to their browser while Opera users have to use Opera 8 to add User Javascript.
Adding the scripts in Firefox is just a matter of clicking on the ‘Install this script’ button which adds the script to the extension. After reloading the page the script becomes active and can be used without restarting the browser.
Some scripts that I listed are rather helpful, they increase the area that displays messages, lets you type messages, remove unwanted objects and let your work comfortably with signatures.
Feel free to mention any script that I might have left out.
Folders 4 Gmail – organize your labels in a folder like hierarchy.
Gmail Air Skin – adds visual effects like transparency to gmail.

Gmail Beautifier – beautifies Gmail.
Gmail Fixed Navigation – the navigation on the left moves with the scrollbar.

Gmail Full Width -removes the advertisement block on the right, giving the content more space.
Gmail Insert HTML Signature – makes it possible to add html signatures per click to your messages.

Gmail Quota Graph – displays the storage quota in a graph.
Gmail Logo Resizer – resized the logo decreasing the height.
Gmail No Footer – removes the Gmail footer.
Gmail – One Click Conversations – allows you to view recent conversations with someone in just one click.

Gmail Signature Float – moves your signature in Gmail to the top of the message rather than the bottom
Mailto compose in Gmail – clicking on a mailto links opens the compose windows in Gmail.
Quick Linkify – makes text links clickable.
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Nice how ever i prefer it nice and clean.
another option
Nice scripts.. I did a similar listing with scripts for GMail and Google Reader here if you’d like to take a look! (linked to my name)
I think GMail Conversation Preview Mod works and looks much better than its one-click conversation counterpart.
Nice list! Stumbled. :-)