Head to Steam to get the Portal Bundle for $1.48! This bundle is normally $19.98. Includes Portal 1 and Portal 2. You can also buy each game separately for only $0.99 each.

Portal 1:

Portal is a single player game from Valve. Set in the mysterious Aperture Science Laboratories, Portal has been called one of the most innovative new games on the horizon and will offer gamers hours of unique gameplay.

Portal 2:

The single-player portion of Portal 2 introduces a cast of dynamic new characters, a host of fresh puzzle elements, and a much larger set of devious test chambers. Players will explore never-before-seen areas of the Aperture Science Labs and be reunited with GLaDOS, the occasionally murderous computer companion who guided them through the original game.