DEVONthink - DEVONtechnologies develops DEVONthink, DEVONagent, and other Mac and iOS apps for document and information management and web research.
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
HoudahSpot - Advanced File Search Utility for Mac OS X
PDF Stacks - PDF Stacks software program helps you organize and manage PDF files, papers and documents. Easily search and read your PDF documents on a PC and MAC
Keep It - Write notes, keep things, and find them again