Audio Hijack Pro can be used to capture audio from almost any program. That makes it hugely popular with (among others) people who produce podcasts and need to record participants from Skype, Google Hangouts, and other communications apps.
Recently, at Macworld/iWorld 2014, iMore’s Rene Ritchie got to talk to Paul Kafasis from Rogue Amoeba about Audio Hijack Pro and why Apple hasn’t opened up iOS to apps like it.