Efficient todo / project / task manager application

I can wholeheartedly recommend Getting Things GNOME!. Its design philosophy is based on David Allen’s work "Getting Things Done: The Art of Stress-Free Productivity". Don’t be misled by its name; it works equally fine with desktop environments other than GNOME. For example, I use it with XFCE.

Apart from the project homepage, you will also find useful information about gtg over at the dedicated GNOME wiki page. This includes keyboard shortcuts and an explanation about how the QuickAdd entry works.

Furthermore, gtg saves all its data in XML-formatted text files. Because of this wise design decision, distributed revision control can easily be implemented. I did so with Hg Mercurial. Now, I can access my task list from any system which does not necessarily need to be connected to the internet at all times. Synchronisation is eventually done over SSH protocol, which guarantees privacy. Merges are handled automatically most of the time.

To get you up to speed with hg, here is a small collection of links to more Mercurial tutorials. Of course, Mercurial becomes superfluous if gtg is used on a single client or LAN.

Finally, Getting Things GNOME! is very easy to learn; it comes seeded with a concise self-contained tutorial in task form!

taskcoach is a GUI based todo manager that has had an active development for a long time.

If you are more into commandline and textfile base ToDo list look at todo.txt (it has some applications that handle the format as well).