Amio Messaging SDK - Develop for multiple messengers using JavaScript

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.

Slack RTM API - Slack's Real Time Messaging API

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

Location SMS - Location SMS is an application that allows the users to locate themselves by sending a simple text ‘Where are you?’ to the phone.

Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.