It can be downloaded for both Linux and Windows. As the case with PPA, if you run Ubuntu, try to install it using PPA, since those versions includes additional features that aren't yet available though the current stable release such as multi language support for instance. With gImageReader you can...
*. Create text files inside PDF documents.
*. Can read directly from various image formats (such as jpeg, BMP, PNG, etc).
*. "Scan" multiple files simultaneously.
*. Check for spelling errors.
These are the main functions. Although it uses the command-line version of the OCR the Tesseract-ocr as said before, what gImageReader does is, it adds an easy to use GUI which makes it a hell lot user friendly :).
Go to this page for downloading the latest builds (for both Windows and Linux).