Windows: Unfortunately there are still some offices and organisations that send scanned documents as image files, instead of searchable PDF documents that you can highlight text in or copy/paste from. If you get one of those documents, GText is a new OCR utility that can help you scan text and extract it so you can copy it into another document.
Once installed, GTText lets you select an image file, load it, and then highlight the area where you'd like the app to translate the image into text. When the translation is complete, a pop-up window will appear that lets you highlight the text and copy it. If the app didn't get it quite right, you can click "try again". GTText recognises BMP, JPG, GIF, TIFF and PNG images, and is available for Windows only.
GTText [Google Code]