The best open source alternative to OmniFocus is Taskwarrior, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 100 alternatives to OmniFocus and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to OmniFocus are fluxday (Free, Open Source), Tracks (Free, Open Source), recordando (Free, Open Source) and Vikunja (Free, Open Source).
Tracks is a web-based application to help you implement David Allen’s Getting Things Done™ methodology. It was built using Ruby on Rails, and comes with a built-in webserver (WEBrick), so that you can run it on your own computer if you like.
recordando (Spanish for remembering) is a program in which to keep your written thoughts This space has these characteristics: It presents a simple organization on screen (like post-it notes) It's safe and secure; it runs on your own computer and not on the Internet ...
TaskLite is built from the ground up to be a command line task manager. It's functionality is similar to Taskwarrior but with a simpler codebase, reduced complexity, faster performance, and more active development.