I'd been wanting to build a big array of mirrors (a la Archimedes) for some time, when I read that Fresnel lenses could be found in rear-projection TVs. Some crafty ebaying later (and some health-endangering demolition), and I was the proud owner of lots of bits of TV, amongst them this fine specimen.
It seems that these things have become significantly more popular recently, but my build was inspired by a magnificent page written by some bored people in the US, who used their array of mirrors to destroy a series of household objects. Unfortunately their site seems to have gone offline.
This thing can melt aluminium cans, and was making short work of the grille we are using in that photo until we defocused it. Apparently it also works fairly well for cooking, in that no-one died of food poisoning, though it does have a nasty tendency to melt cooking utensils or set fire to food.
If you build one of these, remember to buy a welding mask. I dread to think what happened to our retinas before we got that