The best open source alternative to Top Task List is Habitica, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 100 alternatives to Top Task List and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Top Task List are Org mode (Free, Open Source), ToDoList (Free, Open Source), Taskwarrior (Free, Open Source) and Task Coach (Free, Open Source).
DayTasks lets you easily manage adding new tasks, removing tasks, marking tasks complete, archiving completed tasks to your done.txt file, and even filtering and searching through tasks in a simple way that lets you find certain projects or contexts easily.
A self-hosted open source system for the long-term management of personal information. It is a combination of a web-based application intended for desktop use, and an Android mobile reader application.