Elyse - The free version allows up to 30 tags.
GImageReader - gImageReader is a simple Gtk/Qt front-end to the Tesseract OCR Engine.
DEVONthink - DEVONtechnologies develops DEVONthink, DEVONagent, and other Mac and iOS apps for document and information management and web research.
Adobe Scan - Scan anything into a PDF using your mobile device.
Leap - With Leap you find things based on your natural memory of that file.
Tesseract - Tesseract is an optical character recognition engine for various operating systems