there are no AU amps that im aware of under "freeware" for osx
im using logic 9 studio and my AU's for amp emulation are studio devil's guitar amp, amplitube metal, amplitube 1, amplitube 2, amplitube hendrix, amplitube metal, guitar rig 3, guitar rig 4 and logic 9 now comes bundled with its own amp emulator. you also have revalver mk 1, 2 and 3 and line 6's gearbox and podfarm