Firefox

If you're surfing around in FireFox and don't have a terminal fired up, why waste the extra keystrokes or mouse movements just to install a package? If you already know the package name, you can install from your browser simply by using:

apt:<package-name>

For example I want to install the super excellent diagramming software dia, I simply type:

apt:dia

And a GUI pops up much like it does when you select a package and hit apply in Synaptic.