Optical character recognition engine sponsored by Google since 2006.
Ranked in these QuestionsQuestion Ranking
Pro Multiple front-ends available
Multiple GUIs have been developed for Tesseract, including OCRFeeder.
Pro Understands 40 languages
A full list of languages can be found here.
Pro Free, open source and cross-platform
Tesseract is licensed under the Apache with source code available on GitHub. It's available for free on Windows, Linux and OSX.