PHP with GD support required. I plugged in a half-dozen CAPTCHAs but SOLMETRA worked instantly right out of the box. I selected my own background grid and added my own fonts and resized the image -- all very easily done. The image uses random type faces twisted at random angles to thwart robotic readings. You REALLY need a Turing Test on your feedback and contact-me forms to help avoid email header injection attacks. Solmetra is the one I like best. It's the one I use in my form2mail program.