iTunes
QTAACEnc:
http://tmkk.pv.land.to/qtaacenc/QTAACEnc, as the name implies, uses Quicktime's AAC encoder, the same one iTunes uses. You just don't need iTunes open (and qtaacenc exposes more encoder options than iTunes does). Point being, if you've got iTunes installed, you already
have an encoder. qtaacenc makes it easier to access, though - and you can use it with foobar2000 if you so desire.