Meetingbird for Gmail - View your calendar and schedule meetings right from Gmail

Todoist - Todoist is a to-do list that helps you get organized, at work and in life.

Kin Calendar - The simple and connected calendar

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

Calendly for Chrome - Schedule meetings without leaving your inbox

EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.