This revered festival, which is celebrating its 7th year, is the original collaboration beer festival where Colorado Brewers Guild members team up with brewers both near and far and let their imaginations run wild. In the past, there's been collaboration beers brewed with rocks, fresh snow and out-of-the-box styles like sourdough sours and stouts with butter. Two Denver breweries even brewed a beer while rafting down the Platte River.