The hot new wheelset on the market is the Easton EC90 XC 26" carbon fiber wheelset, weighing in (according to Easton) at a mere 1285g per pair, which is about 400g per pair lighter than many road bike wheelsets! There is also a 29er variant weighing in at 1416g per pair.
Get the Easton EC90 XC wheelset at BikeBling.com.
This is the wheelset to get when you build up a really (really) nice mountain bike, like the Moots YBB or similar high-end frame. Or it’s at least what I’d use, which is why I ordered a pair for my on-order Moots YBB. The wheels are a critical part of making a bike feel fabulous.
According to Easton—
The all-new EC90 XC wheelset takes the proven durability of the EA90 XC wheels---and drops half a pound of rotating weight! At a mere 1,285 grams, the EC90 XC is the lightest UST wheelset on the market. And this isn't a typical "race day-only" wheelset. EC90 XC wheels features the superior impact resistance of Easton's proprietary ABC (Armored Ballistic Composite) materials. It's similar to the stuff the military uses in ultra-light body armor and other ballistic applications.
Thanks in part to Armored Ballistic Composite you'll find your wheels spinning straight and true after seasons of abuse. These wheels can shred rough trails all week long and then go on to win world cup XC races on the weekend. UST compatible out of the box takes the stress out of riding tubeless and the hubs are easily convertible to accommodate all of the XC/trail axle configurations.