Here one of the most coolest "on fly" tools that I've ever seen.
Try to load the page many times and try to count the false positive, they are really few !
This easy php page exploits CAPTCHA number thanks to an easy statistical analysis on frequency color per digits. Actually It's quite poor but it runs well and it does the Trick !!!
The main idea is the following:
Here you can find the entire source code.
Via this site.