Valve has made its 2007 shooter-puzzle game titled "Portal" (full-version of the game) available for free download, but until May 24 only.

Here's a video:
YouTube - Portal Teaser Trailer

and if you like it, you can get the game here:
Portal is FREE!

magcreate diay ka ug account para ma-access ang game.
if na-install na ang Steam, download Portal from there.

Enjoy!