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

OmniOutliner - OmniOutliner is a writing app for collecting information, outlining ideas, adding structure to any sort of writing.

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

Dynalist - Dynalist is a web app that lets you break down and organize your thoughts in the format of lists.

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

Outlinely - Indispensable information organization tool for knowledge workers.