My wife will tell you that I have a real obsession with lighting. I just like tinkering around with various lights. For Christmas I got the most expensive little lightbulb I've ever seen, an EverLED. ($40 and no, I'm not getting any kickback from the EverLED people.)
Like the Princeton Tec EOS I blogged about last week, this is a 1 Watt Luxeon LED plus some circuitry but the EverLED crams all the smarts into a little package the size of a conventional PR flange bulb. So I get to mess around with putting the bulb in various housings, trying out various beam patterns and battery packs. The EverLED can run on anything from one to six batteries and it regulates the voltage and current to draw the most from your batteries while giving nice, white light of a constant brightness. When it's drained the batteries, it just winks out and you really don't get any warning, so you want to make sure you've got backup batteries and something handy like a helmet light but this really is a nifty device. Some pretty cheap flashlights and inexpensive halogen bike lights actually have pretty good reflectors and this very expensive bulb lets you tinker to your hearts content.