In a comment to our recent PDF review, Gary let us know about about Skim, a new open source project to produce a tool for reading and marking up PDFs. Skim already has a number of interesting tools, allowing you to:

• embed notes (both shorter notes that appear over the PDF, and anchored notes which are marked by a speech bubble icon that leads to a separate window)
• add circles and boxes
  
• if the text in the PDF is selectable, highlight, strike-through, and underline.
  

Skim even has a full screen mode, as is popular with the kids these days. For a version 0.2 release there's already a lot of functionality here, which bodes well for the future (though I have to say the keyboard shortcuts for notes and markup are insane). Unlike Papers, which we mentioned before, Skim doesn't seem to be intended as a PDF library manager, but rather an individual PDF markup tool. It is available now as a free download from SourceForge.