Leap - With Leap you find things based on your natural memory of that file.
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
DEVONthink - DEVONtechnologies develops DEVONthink, DEVONagent, and other Mac and iOS apps for document and information management and web research.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Elyse - The free version allows up to 30 tags.
EndNote - Accelerate Your Research. Save time, stay organized, collaborate with colleagues and get published with EndNote 20.