For all you Ubuntu/Debian users out there
sudo dpkg -i steam.deb
if some packages are required but aren't already installed, type:
sudo apt-get -f install
sudo dpkg -i /home/*Your_username*/steam.deb
When you execute the application you'll of course be told to log in and verify your account. After that, you'll see your account has not been authorized and you can therefore not use Steam.
To get around this little "check" of Valve's, just type steam://
in the url-bar of your browser and log in.
Enjoy using Steam