Maybe I’m a little late noticing this, but the Labs option once you’re in your gmail settings:
I was using the Better Gmail plugin from LifeHacker for a while before Google upgraded gmail. I enjoyed the fact that my signature was going to be before the email text I was replying. Well, that is offered on the “Labs” tab now with a whole slue of other features. Don’t get me wrong, I still use the new Better Gmail2 plugin to ensure https is used when reading my mail.
The calendar gadget is another favorite because my wife and I use it a lot. We have calendars for each of our sons and have their prespective doctor’s appointments and school related events on there. It’s nice seeing all that information on the side of my mail.
Good job Google.
Update: The new gmail skins offered by the Better Gmail 2 plugin are sweet, check out this redesign one: