WatchOCR is free OCR server for PDFs. WatchOCR uses cuneiform, and exactimage to create text searchable PDFs from image only PDFs and Tiffs. WatchOCR also includes a barcode feature that allows files to be renamed and placed in a directory structure based on information included in a barcode. Using the web interface, WatchOCR can be remotely configured to monitor a watched folder for newly scanned PDFs for OCR conversion. WatchOCR is availble in .deb format for Ubuntu and as a preconfigured LiveCD.
If you like the functionality of WatchOCR but don't want to hassle with installing/configuring it, try PdfCubed.com
If not otherwise specified, this software on the falls under the GNU GENERAL PUBLIC LICENSE. Similar to other Open Source licenses, this means that you can copy, modify, redistribute it without restrictions, as long as the recipient receives the same license. The source code of the standard packages on the CD are available from their respective original providers (for example on the FTP servers at Debian). Special components such as the WatchOCR program and scripts are available on the CD and via .deb files.