DxO Viewpoint is a useful, if extremely limited, app to have on one's computer. Its main purpose is to correct wide angle lens distortion and straighten perspective, and it manages to perform these functions very well and then auto-crop the result to retain as much of the original image as possible. (Note that DxO's product site also claims the app is capable of achieving artistic effects, but in my opinion those shown can be more easily done with other commonly used apps.)
Living in NYC, I am often forced when street shooting to tilt the camera upward in order to take in architectural details. Viewpoint does a nice job of fixing the readily apparent tilt that occurs as a result. This alone justified for me the sale price of $49 I paid for the app.