This is something I made a while back to see how small I could make stuff. It's a surprisingly bright little torch using two 1.8mm SuperBright White LEDs, all packed into a standard 2x6 lego brick (11x14x42mm). It is even still possible to use it as lego, as I discovered when parts of the lego car I was making recently started mysteriously lighting up. This isn't a full instructable as I don't have photos of the build, but if there's enough people asking about it I could probably make one.
EDIT: I have since updated this project to use a 4mm PTM switch, LR44 batteries and copper tape, which makes it a lot more reliable as it solves all the dodgy connection issues the old version had. Please comment or message any questions :)