There are many applications available outside of Nokia Store, which by default can't be installed. Usually those application are called Debian installation files and look the following way: application-name.deb. It's more difficult to install them than a Nokia Store application - but don't worry, it's still pretty easy.
First of all you need to allow your phone to install 3rd party applications (IMPORTANT).
1. Go to: Settings/Applications/Installations
2. Switch on: Allow installations from non-Store source.
This way you can install N9 Debian-files (application.deb) from any source. There are different ways to get them onto your phone:
a. Download .deb file straight to your phone (installation starts by clicking on downloaded file) - this is the easiest way.
b. Download .deb file on your computer and send it to your mobile via Bluetooth (installation should start by itself - else click once on the transferred file and wait)
c. Download .deb file on your computer and transfer it via mass-storage mode to the N9 - open the .deb file with a file manager like Filebox and wait for the installation.
Advantage: Non-NokiaStore applications, beta-version
Disadvantage: Since such applications are not approved by Nokia, they can be a security issue for your phone or might cause malfunctions. Be always careful what you download! If you are not sure, please feel free to ask us.