Are you still feeling bad about that whole Weighted Companion Cube incident? Still pining for it's friendly, rounded corners? A new mod promises to take you back in time to before the events of the original Portal, when your little inert buddy was still free to.. remain motionless.
And what a mod - Portal: Prelude is huge, actually bigger than the original game. It has 8 new chapters, 19 test chambers, a new plot and over 400 lines of dialogue (with English and French subtitles!).
You will need Steam, a copy of Portal and possibly some other bits and bobs like Counter Strike Source - the developers have, rather endearingly, forgotten.