http://store.steampowered.com/freeportal/

If you haven't got the game, now is the time. It's really clever, and isn't too demanding on older PC's.

What is the catch?

No catch. Steam has made it onto Apple OSX, so Valve is celebrating by making Portal free up until the 24th or something. Whether you have to pay for it afterwards, I don't know, but it's not too hard to pass the game in a couple days.

yeah, some of the "trials" can be hard the first time you try them, but once you figure out timing with some of it, it's a really fun game, and challenging.