Should you enjoy Portal on the consoles and PC, then you'll love Portals 2D (by Weesals) on your Windows Phone. Taking an interesting 2D birds-eye route, this indie title feels like a unique game due to the different camera angle from 1st person to the clouds. The concept is exactly the same with cubes, portals, lasers, a portal gun and the requirement of a quick thinking mind.
The most intriguing feature of this Portals 2D is not the superb graphics (vaguely reminiscent of GTA China Town Wars?), but the price - it's available for absolutely nothing until February 7th. You can download Portals 2D from the Marketplace. Note that there are two entries for this app, be sure to download the one we link to.
Thanks gh8421 for the tip!