You can try using 'NitroTask'. It is a simple, nice, and very useful To do list management apps.
It has these features, I was looking for a long time:
- Simple addition and deletion of task
- Theme support, include 5 themes. (awesome feature).
- Task synchronization with Dropbox and Ubuntu one (Cool!!)
- Multi Language support
- Sort by Title, Date or Priority. (very handy if you have a long list)
- Custom default background image support. (Can be used for killing boredom)
To install it, use these command by opening a terminal using Ctrl + Alt + T shortcut.
sudo add-apt-repository ppa:cooperjona/nitrotasks
sudo apt-get update
sudo apt-get install nitrotasks
This is currently available for Ubuntu 11.10, 12.04 and 12.10
I think, you will enjoy this software.
Here is a screenshot below: