Open source scanning software that corrects skewed scans so they align properly.
unpaper is a post-processing tool for scanned sheets of paper, especially for book pages that have been scanned from previously created photocopies. The main purpose is to make scanned book pages better readable on screen after conversion to PDF. Additionally, unpaper might be useful to enhance the quality of scanned pages before performing optical character recognition (OCR). unpaper tries to clean scanned images by removing dark edges that appeared through scanning or copying on areas outside the actual page content (e.g. dark areas between the left-hand-side and the right-hand-side of a double- sided book-page scan). The program also tries to detect disaligned centering and rotation of pages and will automatically straighten each page by rotating it to the correct angle. This process is called “deskewing”. Note that the automatic processing will sometimes fail. It is always a good idea to manually control the results of unpaper and adjust the parameter settings according to the requirements of the input. Each processing step can also be disabled individually for each sheet. Input and output files can be in either .pbm or .pgm format, as also used by the Linux scanning tools scanimage and scanadf. Conversion to PDF can e.g. be achieved with the Linux tools pgm2tiff, tiffcp and tiff2pdf.
(Via reddit)