....A 13 square foot magnifying glass!
Seriously. A solid glass lens that size would be silly, but instead we can use a 4 foot wide Fresnel lens. You know, those clear, flat things with the ridges, you find them on overhead projectors and rear windows on some buses? The idea is pretty simple: a Fresnel lens is just a normal curved lens chopped into thousands of little rings, but just as effective.
This instructable (my second) chronicles my progress over the last month or so on this Fresnel deathray. Each step was figured out in real time, but the general idea is this: once you have your giant Fresnel lens, all that remains is to build a frame to keep it straight, and hold perpendicular to the sun. While you can stop here and enjoy the blinding energy of the nickel-sized spot you get at the focus, I went further and attempted to collimate the light into a straight beam. I ordered a focusing lens online and constructed a scaffold to hold it in place, but ultimately found the Fresnel lens to have imperfections standing in the way of proper functionality.
Disclaimer: This device is extremely dangerous, and will INSTANTLY set things on fire! It's extremely cool, but I'm not responsible for anything that happens if you decide to ignite yourself, your house, the forest, or anything else. Also, if you decide to skip the eye protection step, I hope you like braille.