Luxeon Star
It's a high-brightness LED. Typical LEDs (such as 5mm Nichias) are rated around 70 milliwatts. Lumileds, a joint venture between Philips and Agilent I believe, makes 1 watt and 5 watt LED emitters, known as Luxeon Stars.
A 5W cyan is BRIGHT. And in a 6D Mag it'll last a long time.
5W whites are almost impossible to come by. There are currently two known sources, both with limited quantity: Spares from ElektroLumens' group buy ($35, very limited quantity), and ordering from Arc Flashlight ($45 + shipping). These are NOT drop-in replacements, you'll need to do some soldering to get an LS running in a Mag, and you'll need to get some sort of heatsink. (Supposedly Lambda on these forums has some, I'm awaiting an email back from him.)
You can get premade 5W drop-ins for the 6D from
www.inretech.com, although at a price... Inretech has no single-emitter 5W mods, just 3 and 6 emitter mods, which are expensive. ($200 for a 3-emitter white.)