GPG4Browsers - Recurity Labs
http://gpg4browsers.recurity.com
GPG4Browsers is a prototype implementation of the OpenPGP Message Format [RFC 4880]. The implementation is currently written as Chrome Browser Extension with a Googlemail integration for encrypting, decrypting or signing emails.
Gmail Conversation Preview Bubbles [persistent.info]
http://persistent.info/archives/2005/08/20/gmail-preview-bubbles
"Want preview bubbles for conversations in Gmail, as shown in the screenshot on the left? Then install the Gmail Conversation Preview Greasemonkey script."
Skinning Gmail with a Custom Stylesheet [persistent.info]
http://persistent.info/archives/2004/10/05/gmail-skinning
Not all that thrilled with GMail, but this rocks.
Gmail Agent API - Mail Notifier - Address Book Importer * Johnvey.com
http://johnvey.com/features/gmailapi
"There are two distinct components here: an open source Gmail API written for the .NET framework, and a proof of concept Windows application built on top of that API that provides basic remote Gmail functions."