Tesseract - Tesseract is an optical character recognition engine for various operating systems
Okular - Okular is a universal document viewer based developed by KDE.
Onlineocr.net - Free Online OCR service allows you to convert PDF document to MS Word file, scanned images to editable text formats and extract text from JPEG/TIFF/BMP files
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
Prizmo - Prizmo is a scanning application for Mac with Optical Character Recognition (OCR) in over 40 languages with powerful editing capability, text-to-speech, and iCloud support.
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.