Focuster - Ditch your todo list for an intelligent scheduler
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Taskbook - Like Trello but for the Terminal
Agenda - A date-focused note taking app for both planning and documenting your projects.
Nirvana - Nirvana is a web-based GTD task manager.
ToDoBook - Turn your Facebook (& other social media) News feed into a to-do-list.