The Powder Alliance has updated their website for 2015-16. The new news is that Whitewater in B.C., Canada was added as the 14th partner. Silver Star in BC was added last season.
The better known places include Snowbasin, Bridger Bowl, and Schweitzer.
Worth checking the FAQs at the bottom of the Powder Alliance home page if you are unfamiliar with how the Powder Alliance works. It's not like the Mountain Collective Pass. To take advantage of the benefits, must buy an "anytime season pass" from one of the 14 ski areas. Then can get up to 3 free days at any or all of the other 13 places, which are not exactly close to each other. Each place sets their own requirements and blackout dates. In general, free days needs to be Sun-Fri, so no free Saturdays.
Q) How much does the Powder Alliance pass cost?
A) Nothing. The benefits come free when you purchase a top-tier, anytime season pass to any one of the Powder Alliance resorts.