Gmail Skins is a Firefox extension that lets you customize Gmail interface.
* Change the colour scheme of your inbox
* Insert smileys/emoticons and images in to your emails
* Make the navigation (Inbox, Starred, Sent Mail) horizontal
* Zebra stripes on the inbox (alternating colors for rows)
* Change the attachment paperclip to an icon indicating the file type of attachment
* Hide invite panel, page footer
* Hide your email address at the top - for privacy issues
The skins don't look very pretty, but they disrupt a little the monotonous Gmail look.
Skinning Gmail with user-defined CSS page