Skim is a PDF reader and note-taker for OS X. It is designed to help you read and annotate scientific papers in PDF, but is also great for viewing any PDF file.

Features:

• Viewing PDFs
• Adding and editing notes
• Highlighting important text, including one-swipe highlight modes
• Making "snapshots" for easy reference
• Navigation using table of contents or thumbnails, with visual history
• View all your notes and highlights
• Convenient reading in full screen
• Giving powerful presentations, with built-in transitions
• Handy preview of internal links
• Focus using a reading bar
• Magnification tool
• Smart cropping tools
• Extensive AppleScript support
• Bookmarks
• Saving passwords in Keychain
• Export notes as text
• Automatic download of remote PDFs
• Support for Apple Remote Control
• Interaction with LaTeX, SyncTeX, and PDFSync
• Integration with BibDesk and other third party applications
• Spotlight support
• Highly customizable
• And much more...

Requirements:
OS X 10.6+
Intel

Also available is:
Universal Binary version 1.3.22 for OS X 10.5