Vabotu - More than slack. More than Trello. FREE.
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
Do - Do helps you run productive meetings. Do work you love.
Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.
Kurator - The premier Chrome extension for savvy researchers and publishers alike. Curate your interest and publish to your website, newsletter and social media sites directly from the browser.
Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.