Athennian VS EssentialPIM

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.

CherryTree - A hierarchical note taking application, featuring rich text and syntax highlighting, storing data in a single xml or sqlite file.

Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.

GTDNext - About Us.

DropTask - DropTask is a workspace to manage projects, tasks and to-dos both individually or as a team.