Datomic
MySQL
MarkLogic Server
Valentina Server
Firestore
Datahike
Matisse
Oracle TimesTen
Task Coach
Todoist
Remember The Milk
EssentialPIM
Todo.txt
Things
TickTick
Google Tasks
Datomic
Task CoachMySQL - The world's most popular open source database
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
MarkLogic Server - MarkLogic Server is a multi-model database that has both NoSQL and trusted enterprise data management capabilities.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Valentina Server - Valentina Server is 3 in 1: Valentina DB Server / SQLite Server / Report Server
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.