Earlier this year, I found some uncommon LEDs at Clas Ohlson: Large bright 10mm LEDs (note that the link takes you to selection of different kinds; I got the “ultra white”). As seen from the pictures below, it is unusually large, compared to the 5mm LEDs which are most common. At 5000 mcd it is also very bright, and can easily be used as a flash light. Its forward voltage is 3.2 V, so with three AA batteries and a ~68 Ohm resistor, you’re all set.

I used a macro lens to take some of the pictures, and I found this one particularly interesting (make sure you zoom in 1:1). You can very clearly see the wire bond between the anvil and post. With most LEDs, this is usually too small to get a glimpse of. In this picture, you can get an idea of its cone. Like most LEDs, it is very directional, and thus flash-light idea.