Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
Push It - Simple location aware messaging
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Toodledo - Toodledo provides a set of tools to increase the productivity.
Scheduled tasks - GNOME GUI for configuring a users' cron (automatic jobs). Some of its features are:
vim-taskwarrior - a vim interface for taskwarrior