“groovy mother...”

This I like a lot — CAPTCHAs partially made up of text that couldn’t be OCR’d successfully (so bots are unlikely to crack it), the solving of which in turn helps the Internet Archive digitize books.

SpamAssassin plugin which OCRs the images attached to image-only mails to work out if they contain text like V14GRA. I imagine this is massively CPU-intense, but it might be worth looking at.