Robin Good: FingerPDF is a free Mac app which allows you to collect and organize your PDF document library, to search instantaneously and to analyze, read, and summarize content easily.
FingerPDF can also collect and store any web page (via a dedicated bookmarklet) or Mac document as a PDF inside your library making it available for indexing and future use.
Stored PDFs can be organized in folders, and can be easily tagged.
From the official site: "FingerPDF is a Mac PDF Organizer which allows you to quickly find and organize your PDFs. It is well suited for people that need frequent and rapid access to their books, articles and manuals.
Preview your PDF files, highlight sentences and take notes. Review and export your comments."
Check out this video demo: http://youtu.be/jVfnWH9BlN0
More info: http://www.fingerpdf.com/