LaDiDa: Reverse Karaoke For The iPhone

Think of the LaDiDa iPhone app as your own instant backup band with auto-tune thrown in to help your vocals. "Sing into the iPhone over a metronomic beat, and the software figures out what key you're in, what mode is being used (major, minor, mixolydian, etc.), whether song sections are being repeated, and what chord progressions might work well underneath the melody line" is how ars technica describes it. Here's a demo:

Video: Nerd Woos Dream Girl Using LaDiDa App: