It seems like a pretty simple sound to make, and it's sort of their trademark now.

Most older video games just sampled other, existing stuff, because sound design is hard. Valve have always had excellent sound design though.

It's funny, I'd been on here for years and never played a single Valve game except for the first Half-Life which I think were days that predated poe-red. But I got The Orange Box for my little brother for christmas this year and felt bad because I turned out staying up and beating it after he went to sleep. I'm not a huge gamer anymore, but Portal in particular is one of the few games that actually makes me laugh and feel other emotions, unlike most games' forced attempts at humor or drama that I've seen. I know I'm preaching to the choir on this one, but that little sound means as much to me as the weighted companion cube itself.