Editor: MacPips Loopifier lets you modify the loop's start and end zero crossing points while you listen.
Simply record a few repeats of a looping phrase into Garageband, export your best version to an audio file, drop it onto Loopifier and adjust the sliders until the phrase loops without clicks or pops.
NewVersion 1.1: Loopifier can now normalise the loop on export, and the normalisation level is user-definable.