I've been using the Google PhotoScan app on my Android phone since discovering it some time ago. It was originally designed to photograph pictures behind glass and eliminate "hotspots" and reflections. This feature makes the app particularly useful for "scanning" old photos and documents that have curled corners. Place your photo or document on a flat surface and cover it with a clean sheet of glass (get one at your local hardware or craft store, or take one out of an unused picture frame.) The PhotoScan app will automatically adjust the lighting, remove reflections and hotspots, and crop the image to only include your document or photo.
Additional tip: Make sure the flat surface you're using is a different color than your documents and photos. For instance, if you're photograping documents on white paper place them on a wooden table, or if your photographs have a sepia tone place them on a white cutting board before laying the glass pane over them. The color contrast between the document and work surface allows the PhotoScan app to find the edges for cropping and making small adjustments to correctly align the document to be straight.