Those of you with brains like computers will probably have worked out that you can use your Portal gun in other Source-powered games like Half-Life 2 and its pair of episodes.
And the benefits are obviously enormous, as you can drop cans of paint on Combine forces from lots of inventive places and they will have no idea how you are doing it.
All you have to do is roll up your sleeves and merge your Half-Life 2 maps and scenes with your Portal content. Then, when you fire up Portal again you will be able to pull down the developer console and pick a map to use your new gun on.
Primotech has a step-by-step guide on how to do this, and remember that it is unsupported by Valve so you will need to back up all of your files before you start tinkering with things.
It drags up the question of whether we will see the Portal gun in a Half-Life 2 adventure sometime soon, something Valve has previously denied as it would basically be too complicated to regulate.