Enhance GMail with Greasemonkey

Gmail is a great web email service, in my opinion the best out there, and Google have recently made some updates to improve it further. However, there are still some features missing, such as HTML signature. Fortunately, though, there is a Firefox extension called Greasemonkey which allows customisation of a web page through the use of Javascript scripts. The customisations range from changes in the layout (such as removing flash and iframes to help clean up the page) or adding/changing functionality (such as using a web mail service for handling mailto: links).

There are several great scripts available for Gmail. The following are my favourites:

GMail Addons - allows viewing of Google Calendar while within Gmail and provides an improved Google Chat.
Ehance Google Cal (requires the above Gmail Addons) - cleans up the Google Calendar page when viewing it within Gmail.
Mailto to Webmail - allows the use of a webmail service (GMail, Hotmail, Yahoo! Mail, etc) as the browser default email client instead of Outlook or Thunderbird.
De-ad gmail - removes the adverts from the right of emails in gmail.
Gmail FavIcon Alerts - changes the favicon to notify of new message(s).
Gmail - Insert HTML Signature - allows a HTML signature to be inserted when composing an email in Gmail.
Secure Pro and also part 2 - forces Gmail (and some other services such as Facebook and Google Docs) to use encryption (ssl/https)