InTechSpot  

Gmail Skins is a Firefox extension that lets you change the look of your Gmail inbox (amongst some other things).
It adds the following features:
Change the colour/skin of your inbox
Insert smileys/emoticons and images in to your emails
Make the navigation (Inbox, Starred, Sent Mail, etc) horizontal
Fix the navigation in place so that you don't have to scroll to the top of the page to see it
Zebra stripes on mailbox - pretty!
Change the attachment paperclip (on inbox) to an icon indicating the type of attachment
Hide various page elements (invite panel, page footer, your email address from the top right of inbox - may be useful for public computers)