These brake pads from Discobrakes are for Shimano XTR, XT, SLX and LX disc brakes as well as Hygia SLP and Elite disc brakes. They are aluminium backed and weigh only 8g for a pair. The compound is semi-metallic / non-asbestos.
Note: my scales show the pads weighing 7g, but the scale is only accurate to +/- 1g, so factor in the pads actually weighing 8g as claimed by the manufacturer.
If you're interested in the weights of a variety of XTR brake pads then have a look at the article on Shimano XTR brake pad weights .
How do these ultralight XTR brake pads perform?
Performance of this pads is pretty much indistinguishable from those of the standard metal backed semi-metallic Hygia pads. The aluminium backing that makes these so light seems to have no beneficial or adverse effects on brake performance. Time will tell whether they wear quicker than the stock pads.
Update - October 2011
I've ran through 4 sets of these pads along with the Hygia stock rotors and Alligator Aries rotors.
In the dry, these pads are great, but I've found that in the wet when combined with the Alligator Aries rotors they have a fairly short life. Not only do they wear fast, but large chunks of pad material seems to get dislodged.