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Tesseract is an Open Source OCR engine, available under the Apache 2.0 license. It can be used directly, or (for programmers) using an API. It supports a wide variety of languages.
Tesseract doesn't have a built-in GUI, but there are several available from the 3rdParty page.
There are two parts to install, the engine itself, and the training data for a language.
Tesseract is available directly from many Linux distributions. The package is generally called 'tesseract' or 'tesseract-ocr' - search your distribution's repositories to find it. Packages are also generally available for language training data (search the repositories,) but if not you will need to download the appropriate training data, unpack it, and copy the .traineddata file into the 'tessdata' directory, probably /usr/share/tesseract-ocr/tessdata or /usr/share/tessdata.
If Tesseract isn't available for your distribution, or you want to use a newer version than they offer, you can compile your own. Note that older versions of Tesseract only supported processing .tiff files.
The easiest way to install Tesseract is with MacPorts. Once it is installed, you can install Tesseract by running the command sudo port install tesseract, and any language with sudo port install tesseract-<langcode>. List of available langcodes can be found on MacPorts tesseract page.
Other option is to install tesseract using Homebrew with the command:
brew install tesseract
An installer is available for Windows from our download page. This includes the English training data.
If you want to use another language, download the appropriate training data, unpack it using 7-zip, and copy the .traineddata file into the 'tessdata' directory, probably C:\Program Files\Tesseract OCR\tessdata.
Tesseract may work on more exotic platforms too. You can either try compiling it yourself, or take a look at the list of other projects using Tesseract.
Tesseract is a command-line program, so first open a terminal or command prompt. The command is used like this:
tesseract imagename outputbase [-l lang] [-psm pagesegmode] [configfile...]
So basic usage to do OCR on an image called 'myscan.png' and save the result to 'out.txt' would be:
tesseract myscan.png out
Or to do the same with German:
tesseract myscan.png out -l deu
Tesseract also includes a hOCR mode, which produces a special HTML file with the coordinates of each word. This can be used to create a searchable pdf, using a tool such as Hocr2PDF. To use it, use the 'hocr' config option, like this:
tesseract myscan.png out hocr
More information about the various options is available in the Tesseract manpage.
Tesseract has been trained for many languages, check for your language on the Downloads page. It can also be trained to support other languages and scripts; for more details see TrainingTesseract3.
Tesseract can also be used in your own project, under the terms of the Apache License 2.0. It has a fully featured API, and can be compiled for a variety of targets including Android and the iPhone. See the 3rdParty page for a sample of what has been done with it.
Also, it's free software, so if you want to pitch in and help, please do! If you find a bug and fix it yourself, the best thing to do is to attach the patch to your bug report in the Issues List
First read the Wiki, particularly the FAQ to see if your problem is addressed there. If not, search the the Tesseract user forum or the Tesseract developer forum, and if you still can't find what you need, please ask us there.