In #129 I posted about a two-transistor boost circuit. This is an even simpler circuit - very common over around the internet and known as the archetypal "joule thief". The smallest I've seen is Eric Wasatonic's micro version.

I stuffed my circuit into a AA battery holder and it works great as a night light, powered by either a single somewhat-depleted AA or AAA battery.

As always, all notes, schematics and code are in the Little Arduino Projects repo on GitHub.