Available in both 26" and 29" versions, this 'race only' tyre from Bontrager combines the speed of their older XR1 with a bit more on the sides for those who actually like to go round corners. 'XR' denotes the intended application and '0' the amount of grip on a scale of '0' to '4'. In other words, cross country riding with minimal rolling resistance.
Out of the box this is a seriously light tyre, sub 500g even in the 29" x 1.9" version with paper thin sidewalls, minimal tread and a super-soft rubber compound. It's not designed to be run tubeless, but that didn't stop me getting it to air up tube free fairly easily on both Bontrager and Stan's rims with some sealant. The thin sidewalls and tall nature of the tyre means running it below 30psi results in lots of squirm, stick a bit more air in and those attributes produce a tyre that starts to feel like a tub, zinging along nicely, giving a comfy ride with the supple carcass conforming to the ground and offering more grip than expected.
Downsides? Leave it on your bike for everyday use and it'll wear down quick. The durability also means caution is required on rocky trails (although I've got away with using it at Dalby Forest so far). Also the thin carcass means a trailside tubeless repair isn't possible (but then you aren't supposed to use it without an inner tube)...
A slightly wider, slightly tougher, tubless version would be ace, but that misses the point of a tyre that in the range of riding conditions it's intended for is rocket-ship fast.