Just when you thought companies had run out of places to put ads online, a start-up called Solve Mediais bringing them somewhere new -- the boxes of letters you sometimes have to type to get access to sites.

Those boxes, called captchas, are intended to block malicious programs from breaking into sites and typically are used to restrict access to things like email accounts. Computers have a tough time reading the text, which in traditional captchas is squiggly, even if humans can decipher it and re-type it.

On...