CP3S said:
they seem to have fallen into the habit (starting way back with Counter-Strike, actuall of scoy)ping out small developers with promising and original ideas, then buying them out, employing their developers, and allowing them to run with these ideas in ways they previously never would have been able to dream of. Which is actually really cool.

They actually did this with Portal 2 as well. The different gels were invented by a team at Digipen University in a game called Tag: The Power of Paint, who also got hired by Valve.

It's pretty interesting that pretty much all of the people responsible for the Portal games were graduates of DigiPen University (which is also based in Washington state).