Human Input Validation
This is a handy image script for generating those random keys you have to type in before registering or submitting a form. You see these all over the place these days. The idea is to generate an image that is difficult enough for someone to write a script to interpret that they don't bother. You would do this if you don't want people registering a lot of accounts autonomously or whatever. I think this mostly applies to script kiddies that aren't capable of writing an OCR script.
This script utilizes sessions to store the image text when the image is loaded. It compares the session text to the text you submit before the image is loaded again. If the image is reloaded in the same session in a different window, the input will not match.