A friend pointed me at a Scientific American article today, titled “Baffling the Bots“. Its a fun read, I suppose. It sort-of credits Yahoo as having pioneered this stuff in 2000. But we totally did this at Remarq in 1998/1999.
The reason we had to do it was because we had tons of images on our site, and people were sending bots to go find them all. This ate up a fair amount of bandwidth. So, we just required users to type in the number in the picture before they could view pictures, and they had to redo this every 50 pictures or so. Interestingly, we just displayed a simple 3 digit number. The implementation was cheap – we pre-generated the numbers and actually only ever displayed about 50 different numbers. As far as we knew, though, it worked 🙂