DEVONthink - DEVONtechnologies develops DEVONthink, DEVONagent, and other Mac and iOS apps for document and information management and web research.
dtsearch - dtSearch - The Smart Choice for Text Retrieval since 1991.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
DocFetcher - DocFetcher is a portable German/English open source desktop search application.
HoudahSpot - Advanced File Search Utility for Mac OS X
SearchMyFiles - Alternative to the standard Search For Files And Folders module of Windows. Duplicates search is also supported.