Heres an Instruction to make your own Portal Companion Cube Keychain with a 3D Printer.

Step 1: First: Print.

Ull need a 3D Printer and some Paint.

Print this 3D Model: http://www.thingiverse.com/thing:23899/#files
(Thanks to CarryTheWhat for the Model itself)

Step 2: Paint the Deepest First.

Paint the deepest places first.

I used Acrylpaint.

Step 3: Overpaint the Paint Like 3-5 Times

Overpaint the Paint like 3-5 Times until you cant see it anymore.

Be gentle and do it like you would paint over a plate without going under, like you Paint the air over the deeper Places.
(Argh, man. You know what i mean)

Step 4: Adding Lines.

Now you need to add the small Lines in the Middle.

I used a TOOTHPICK to do it.
Use something like that and your finished.

Mine is like 4 Months old know.
Thats why it have a nice used-Touch.