Skim (Free).

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.

Stop printing and start skimming.

Features:

Viewing PDFs
Adding and editing notes
Highlighting important text
Making "snapshots" for easy reference
Navigation using table of contents or thumbnails
View all your notes and highlights
Convenient reading in full screen
Giving powerful presentations
Handy preview of internal links
Focus using a reading bar
Magnification tool
Extensive AppleScript supportBookmarks
Support for Apple Remote Control
Interaction with LaTeX and PDFSync
Spotlight support
Highly customizable
And much more...

Get it here.