Mixmax is a free Chrome extension which upgrades Gmail with a host of useful extras. You get instant visual previews for YouTube videos, Google Maps, Twitter timelines, Pinterest pins, and other links and attachments.
It’s easy to create quick polls and surveys. The user clicks one of the choices in the message and you can review the results later.
“Instant scheduling” allows you to set up meetings via Google Calendar, email everyone involved with available times, and have you confirm them directly in the message. The free Mixmax limits you to 10 meetings a month, but that’s more than enough to be useful.
Email tracking aims to show you if and when your message was opened. It uses read receipts, though, which is good for privacy (no sneaky embedded graphics to track people without their knowledge), but bad for effectiveness (if the client ignores read requests then you won’t see anything).
A "Send later" is fortunately more reliable, enabling up to 10 messages a month to be scheduled for some later date.
And the free Mixmax can save up to 10 common emails as Templates for easy reuse later.
Mixmax worked well for us, and if you can live with the free version’s "Sent by Mixmax" signature -- or don’t mind the $9/ month fee to remove it, and the other restrictions -- then it’s a good choice.