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.

Pusher Beams - Send push notifications programmatically at scale

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

Snowball Smart Notifications - Get the notifications you want and none of the rest

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

Engagespot - Engagespot is the easiest way to build your product's multi-channel notification infrastructure. Deliver In-App, Email, Push, SMS, WhatsApp and chat notifications from your app to your users with a single API.