iTunes reports: [edited]
Skitch lets you communicate your feedback and important ideas in an instant. Use Skitch to mark up a PDF, snap your screen, annotate a photo, or draw something new. Share your thoughts with friends or co-workers. Annotate images and PDFs with arrows, stamps, text, shapes, and more. Getting everything done just right has never been easier.
Free. System requirements: OS X 10.7.4 or later, 64-bit processor.