Ever have the need to change the program or application that a file type is associated to in GNOME? If so, you can easily make the modifications in Ubuntu (or any Linux distribution).
To do so, open up Nautilis (click on Places in the top panel menu then open your home directory) and find the file type that you want to change the association.
In the example below, say you wanted to change the default association for .txt files, which is GEDIT to another program. To change it, just right click on the file and select Properties.
Next, select the Open With tab, then select the program from the list by clicking on the radio button next to the program. Click on the Close button to save the setting.
If the program you want to use was not in the list, click on the Add button at the bottom, then in the Add Application dialog window, select the program you want to use from the list.
Notice that you can also click on the arrow icon next to Use a custom command (circled in above screen shot) and browse anywhere on your computer to select a program. You can also add custom command parameters to the program executable when launching the program.
That's it. The next time I double click on a file with a txt extension, Open Office Word Processor will open. Later if I wnat, I can go back and change it to the default aplication which was GEDIT using the above steps.