Can anyone recommend me a good to-do application?

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:

The command line

Let's not forget the commandline for managing tasks

sudo apt-get install task
$ task add some very important task
$ task 

You can also install vit if you want an alternative UI to this list. There are also some command line tools for managing your agenda.

Since Wunderlist native linux client is not maintained anymore good alternative is Wunderlis Package app for Chrome/Chromium (it doesn't require Google Chrome to run to be able to use it).

• The Wunderlist Packaged App also features voice input (you can dictate your to-dos) and has the ability to work offline.

• It's simple and easy to use, supports timed reminders, recurring to-dos (although its recurring feature is definitely lacking), separate reminders from the due date of the task, notes and additional info associated with your to-dos, shared to-dos with others, multiple categories, and more. You can star important tasks (but that's as close to priority as you'll get), and customize the look of the app. It's broad platform support—and its webapp—mean you'll always have access to your to-dos.

Another good alternative is Any.do Package app.

• Breaking things down into "Today", "Tomorrow", "Upcoming", and "Someday" allows you to really make sense of just what needs to happen when.
• Visual priority markings, and notes (which themselves act as sub task lists) makes the app even better. But the real cream of the crop aspects are the "Plan Your Day" daily review of the lists.