Google Calendar gadget (shows a small calender, right inside GMail)
Inbox preview (shows preview of Inbox, while GMail is loading the first time, it is start)
Mail Goggles (Asks you simple maths questions, if you are sending mails at weekend's late night. Reason being obvious. I don't need, still enabled it, just in case.)
Mark as Read Button (Very useful button)
Message translation
Signature tweaks (Places your signature before the quoted text in a reply, and removes the "--" line that appears before signatures.)
Undo Send (VERY VERY Useful, since it gives you 5 sec more before sending message, since you might have forgotten something, and helps you edit the message, before again sending)
Labs feature, which I might try some time later in future:
Hi Guest we just wanted to alert you to a major change in the forum. We will no longer be allowing the posting of outgoing links. Please use the attachment feature to attach media to your posts.