ffmpegX. Just google it. It also has a quick drop down menu option for audio only AAC or something of that sort in its initial window. That's the one I used last time I had to do something of that sort.
I'd agree that Audio Hijack is best for that sort of thing. after you have recorded it, just import it into iTunes and then go to advance menu and click convert to MP3/AAC/Apple Lossless/AIFF (whichever you have your iTunes set to import as)