I just bought my license for the new Audio Hijack 3 from Rogue Amoeba. Wow.
Audio Hijack has always been a stellar app. (For lesser needs, I also really like Rogue Amoeba's Piezo.) What is really impressive about this version is the way they had the guts to scrap the old, perfectly functional user interface and build an entirely new one. What is even more impressive is that the new user interface is so much better than the old. So often, drastic user interface changes result in something clever but unusable. That's not the case here.
Audio Hijack lets you manipulate audio on your Mac with the same aplomb a magician can pull a pigeon out of his pants. Would you like to combine your microphone with an iTunes track into 16-bit AIFF with an onscreen equalizer and a VU meter? No problem. This app isn't for everyone but if you've ever wanted to really work/record/manipulate audio on your Mac, look no further.