I'll start...
Here's a high power LED project I've been dinking with for a while.
It's a constant current boost driver for a 100 watt led, meant to run from a car battery. I'd like to get this driver to the point where I can make and sell some on ebay. The only thing comparable to this are chinese circuits which at their minimum input voltage can't really run these LEDs at full power .
These 100 watt LEDs are a 10x10 array of 1 watt emitters, they require about 33 volts to run and draw around 3 to 3.5 amps, so driving them can be a bit challenging. The price of these has come down a lot; they're only about $8 each on ebay and they produce somewhere around 7000 lumens of light output (double that of a 35w HID headlight).
Since LED are current controlled devices, they should be driven at a certain current, not a certain voltage. They have non-linear voltage-current curves so a small change in voltage can cause a large change in current which can blow them up. Also as they heat up and their 'resistance' changes, constant current control accounts for this automatically.
Here's a picture of the circuitry and the mosfet switching waveform, running at around 155khz in this picture.
Still in development, overall circuit efficiency was 91% last I checked. It's drawing about 10 amps from the battery pack.
Here's the prototype running from a 11.1volt 3 cell LiPo battery pack. A large copper CPU heatsink and cooling fan is used to deal with the ~80watts of heat output (high power LEDs have a luninous efficacy of around 20-30%).
Here's a light output comparison picture. This is 'before' with only ambient lighting.
And after with... This is without any optics, just the bare emitter flooding light.
These would make really good, cheap outdoor lights as you can see.
Optics are available for these though. Here's a 82mm aspheric glass lens and reflector.
These are probably what I'll use when I make an offroad driving light out of one of these.