AmpKit for the iPhone and iPad (Free version with in app purchases for other amp models or loaded up version for $19.99 with just about everything installed already) has been a highly rated and reviewed guitar amp modelling/recording app - and now a version of AmpKit for the Mac has been released.

Includes all the great gear that AmpKit is famous for at no extra charge: 22 faithfully-reproduced amps with 38 separate amp channels, 28 pedals, 28 cabinets and 8 mics.
A true native OS X app, built specifically for OS X Lion and Mountain Lion, with snappy 64-bit performance and a beautiful, immersive user experience.
Even better sounding on the Mac, with improved simulation fidelity to deliver superb tone for both clean and high-gain play.

Effects include multiple overdrive, distortion, fuzz, compression and phaser pedals, plus clean boost, delay, reverb, acoustic simulation, wah, EQ, octaver, tremolo, vibrato, chorus, and flanger.
137 presets to inspire you: from smooth cleans and earthy acoustics to bone-crushing high-gain metal.
Share setups with other Macs, iPhones and iPads. Add setups shared by others right from Mail, Finder, Safari, or AirDrop.
Drag-and-drop to add or rearrange gear, or use keyboard shortcuts, with undo / redo support.
Sophisticated recording capabilities, including non-destructive re-amping so you can change your tone after you record.
Jam along with backing tracks, tune using a high-precision tuner, keep time with a flexible metronome.
The perfect upgrade for GarageBand users looking for a dedicated guitar / bass amp and effects app.
Uses many of the newest features in OS X Lion and Mountain Lion: Multi-Touch gestures, Full Screen mode, Sharing via email or AirDrop, and gesture-enabled Quick Look.