Recently I've been trying to scan lots of photos at once. The problem is each scan takes a long time and some of them are slightly skewed.
I was looking online for a free solution and thought this was free but it isn't:
http://autosplitter.com/
This looks like a good option since it's just a GIMP extension.
http://francoismalan.com/2013/01/how-to-batch-separate-crop-multiple-scanned-photos/