The latest and greatest Gmail Lab is called Sender Time Zone and is perfect for someone like me who has friends and family located in many different time zones.
Now when you enable Sender Time Zone, you'll see a green phone icon next to your contacts that are reachable (it's between 9 a.m. and 6 p.m. their time) and red phones for those who are away from their desk or sleeping. And when you click "show details" you'll be able to see what time the message was sent from your time zone and what time it is for the recipient. Now you can avoid the small talk of asking "so what time is it over there?." Thanks Gmail!