You want free, you got free.
Acid Xpress is the lite version of Sony Acid Pro.
Doesn't have all the functions of the commercial version but still allows the creation of up to ten loop (pre recorded music clips) based tracks.
A pack of free loops can be downloaded from the site.
Need sign up but more Acid compatible free loops from Macloops:
More free loops.
Sony Acid Pro 7, FL Studio, JamTrax, Mixcraft, Dance EJ and Magix are also loop based apps but are commercial not free apart from free trials.
Sibelius and Finale Notepad are music notation programs.
They do not use loops.
Audacity and Wavepad are audio recording and editing apps.
Guitar Pro and Tux guitar are intended for writing guitar music.