We reported on the original version of Capo a while back, and now the folks at SuperMegaUltraGroovy have released version 2 over on their website. The biggest feature in the new release is "a highly detailed spectrogram visualization," which basically means that they've implemented what the first app couldn't do; there's actual chord detection in the software. As you play the song, you can see visualizations of where the notes fall on the scale, and you can lay down chord markers that will automatically mark out your chords. Basically, you can reverse engineer the song.
It's pretty wild. You'll still need some musical insight since the app doesn't always get it perfectly right, and you'll need to know where the chord changes are in order to mark them out. It's impressive, though. I was able to get a basic chord structure out of the first verse of the Decemberists song above, and if I played around with it some more, I bet that I could reproduce the song on the guitar.
You can also have it write out tabs for you, and all of the features of the first version are back; you can mark out choruses and verses on all of your songs, correct pitch and speed, and set up loops to play along with. If you do any amount of playing around with your favorite songs, it's a fun tool to have. Capo 2 is US $49 as a new purchase or US $19 as an upgrade from the first version. It's also available as a free five minute trial download from the site.