TL;DR Email - Send short emails & read emails as fast as you read Facebook
Topicbox - Put team emails into one centralized, searchable place
mlist - Email newsletters, outside of the inbox
InboxSDK - First usable API to write Gmail Apps for 1B active users
Spark Mail - Spark helps you take your inbox under control. Instantly see what’s important and quickly clean up the rest. Spark for Teams allows you to create, discuss, and share email with your colleagues
Front - multi-channel inbox service for collaborative teams