Task Coach

Task Coach is an offline todo or task list management tool. It is available for Ubuntu, other Linux distros and even Android. It just saves a single .tsk XML file that you can place in your Dropbox for access across multiple computer and devices.

To install in Ubuntu:

$ sudo add-apt-repository https://launchpad.net/~taskcoach-developers/+archive/ppa
$ sudo apt-get update
$ sudo apt install taskcoach

Tried with: Task Coach 1.3.36 and Ubuntu 14.04