Dynamic Campaigns by Sigstr - Share the most relevant content in every email
Taskade - Make lists, organize your thoughts, and be inspired to get things done. Taskade is a collaborative space for your tasks.
PushMarket - Automate personalized emails for ecommerce (YC15)
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
InboxSDK - First usable API to write Gmail Apps for 1B active users
Tables by Google - Tables helps teams easily track & automate tasks