Ubuntu users can simply follow these instructions:

 
sudo add-apt-repository ppa:andreas-diesner/garminplugin
sudo apt-get update
sudo apt-get install garminplugin 

Official instructions on how to add ppa:andreas-diesner/garminplugin to the system.

Since November 2012 there is an official debian sid package with the name “garmin-plugin”. Simply install it.

Old instructions: Debian users can download the deb files from the following url and manually install it
http://ppa.launchpad.net/andreas-diesner/garminplugin/ubuntu/pool/main/g/garminplugin/

The plugin will not get detected by Iceweasel since it will be installed into the wrong directory. You can correct this by creating a link in your home directory:

mkdir -p ~/.mozilla/plugins
ln -s /usr/lib/mozilla/plugins/npGarminPlugin.so ~/.mozilla/plugins/npGarminPlugin.so

There is a repository for Fedora, OpenSUSE, Mandriva and CentOS Users. Go to the following website and follow the instructions:

http://software.opensuse.org/download/package?project=home:garminplugin&package=GarminPlugin

Thanks goes to Robert Walker who introduced me into the mysteries of RPM building and Axel Braun who helped me using the openSUSE build system.

José San Juan has created an arch package