OneSignal - Customer engagement platform used by over 1 million developers and marketers; the fastest and most reliable way to send mobile and web push notifications, in-app messages, emails, and SMS.

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.

Gravitec.net - Gravitec.net is a smart web push service that lets you boost subscribers through customizable prompts and automatically target your content to the right audience.

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

Truepush - Unleash limitless possibilities with Truepush and save up to $600 monthly. Free up to 10K Subscribers on all features.

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