Below is a list of Gmail Add-ons that I tried and found quite helpful.

• Tired of looking same old blue-white interface of Gmail then try this firefox add on which can

a)change the colour/skin of your inbox
b)embed Zebra stripes on mailbox
c)Change the attachment paperclip (on inbox) to an icon indicating the type of attachment.
Here is the add-on: Gmail Skins

• And want to turn your Gmail account into a portable hard drive then download Gdisk.
• Googlepedia is a free software extension to the Web browser, Mozilla Firefox, that displays relevant articles from the free Web-based encyclopedia, Wikipedia, on Google search engine results pages. Download link: Googlepedia