Mixmax is a Chrome extension which extends Gmail with a host of productivity-boosting features.
"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.)
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.
You get visual previews for links - YouTube videos, Google Maps, Twitter timelines, Pinterest pins and more - and attachments, convenient Dropbox and Box integration, and the ability to save common emails as templates for reuse later (a maximum of 10 in the free edition).
A "Send later" option schedules messages to be sent at some later date and time.
Mixmax also supports email tracking, showing you if and when your message was opened. Well, maybe - it uses read receipts, which means you won't see anything if your recipients ignore them.
The free Mixmax works well, but does come with a "Sent with Mixmax" signature. Paying $9 a month for the Starter account removes this and the other restrictions, as well as adding better email tracking and priority email support, while the Professional ($19) and Premium ($49) accounts variously support click tracking, smarter scheduling, mail merge, CRM integration and more.
Mixmax offers a strong set of Gmail extensions, and the free version isn't too restricted (although you will have to live with a "Sent by Mixmax" signature).