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.
HoudahSpot - Advanced File Search Utility for Mac OS X
Yojimbo - Yojimbo makes keeping all the small (or even large) bits of information that pour in every day...
Keep It - Write notes, keep things, and find them again
IDocument - Feel powerless of your documents chaos in your Mac?