Installing applications in Ubuntu is getting easier and easier. First there is the command line interface, next is the Synaptic Package Manager, followed by Gnome Installer (Add/Remove). Now there is Appnr, a web based tool where you can search and install applications directly on your Firefox.
Appnr makes use of the apturl protocol to initiate the installation process. Since apturl was included as a dependency for Firefox since Ubuntu 7.10, if you are using a later version than Gutsy, you can simply click and install applications directly from Appnr.com. For other debian based distro, you will have to install apturl:
sudo apt-get install apturl
The navigation in Appnr is easy. On the left are the categories and the respective applications on the right. There is an “Install” button on every application entry and you just click on it to install the particular application.