From the Papers site:
Do you have dozens of PDF files from your favorite scientific articles scattered on your harddrive? Do you also try to desperately organize them by renaming and archiving them in folders? But like the piles of printed articles on your desk, you can’t keep up with all the new papers you download, and despite all your efforts it has become impossible to find that one article.
Finally that all belongs to the past. We’ve been there, trust us, we know. That’s why we wrote Papers, our latest application exclusively for the Mac. Papers will revolutionize the way you deal with scientific papers. Search for papers using PubMed, directly retrieve and archive PDFs, and read and study them all from within Papers, your personal library of Science.
From the Ars Technica article: