http://www.cree.com/products/xlamp7090_xre.asp - for Cree specifications and details on the "XR-E" family, spec sheets are available and highly detailed, under "product documentation" link.
http://www.luxeon.com/products/luxeon/ - for Lux I/III/V/K2/Rebel spec sheets and details, as above, datasheets are extensive and detailed.
Couple of items not completely covered by above posts about the bare LED packages (not counting aftermarket star mountings and such).
The new Rebel is very much an "SMD" (surface mount design) component. It is TINY and best suited to reflow mounting methods (PC board mounting with reflow solder)... VERY poorly suited to hand-soldering. Also, the tiny package causes a very strong "point-source" of heat, if not very well thermally bonded to a heatsink, the die will rapidly heat up (bad).
The Cree is a larger package, although still quite small, it is capable of being hand-soldered if the underside pads are properly isolated from shorting out on a metal heatsink, for instance. The larger ceramic package with it's larger thermal pad underneath is much more favorable to getting heat away from the die, although careful thermal mounting is still a strong consideration with it, it is a bit more flexible for non-solder mounting solutions.
In either of the two above, I personally recommend (as do the manufacturers) direct soldering to a surface in order to speed heat away from the package, the Cree however is commonly "thermal adhesive" mounted by many DIYers with success (including myself, in lower-than-max current setups). However in all my new devices I am reflow soldering the packages to PCB and heat-spreader/heatsink setups to maximize reliability and heat transfer.
Again, EVERYTHING you'd like to know is found in their datasheets on their websites (including more than you'd ever care about for handheld light applications, like how much flat PCB surface area is needed per watt of emitter output in still air in a vertical orientation or a horizontal orientation, etc, compared to forced airflow, yadda yadda).