#1: Checker Plus for Gmail
Summary: Get desktop notifications, read, listen or delete emails without opening Gmail or Inbox by Gmail & easily manage multiple accounts.
#2: Send from Gmail
Summary: Makes Gmail your default email application and provides a button to compose a Gmail message to quickly share a link via email.
Summary: Schedule emails in Gmail to be sent later. Create recurring emails, set reminders and follow up conversations. While there is a free version it does limit you to schedule 10 emails and receive 10 notifications a month. There is a nominal fee for upgrading to unlimited.
#4: WiseStamp - Email Signatures for Gmail
Summary:WiseStamp enables you to share your social profiles and promote information you care about in every email you send.