I'm working on text classification. I have a decent classifier that's especially suited to author identification. I can think of a few good uses for it; the first one I'm trying to commercialize is academic anti-cheating.
You are probably aware of it already, but a lot of universities (mine included) use MOSS (Measure Of Software Similarity) to detect plagiarism in CS classes. Link: http://theory.stanford.edu/~aiken/moss/
I'm targeting the custom essay - services like http://essaymill.com ("our writing, your success"), as well as students paying other students to write their papers.