Apple has updated Garageband for Mac today with the ability to export songs as MP3s, a feature that was removed in the big redesign of Garageband last fall.
Other changes include:
• Improves stability and addresses a number of minor issues
• Adds the option to export songs as an MP3
• Contains multiple enhancements to Accessibility
• Includes 3 new Drummers and drum kits from the rock, songwriter, and R&B genres*
Garageband is available to Mac users for free.
Source: Mac App Store