Sioux City Sarsaparilla is a brand of sarsaparilla soft drink that is comparable to root beer. It is promoted as "The Granddaddy of all Root Beers." The product is generally sold in glass bottles and usually comes in packs of 4 bottles (though it, as with all Sioux City brand soft drinks, is also available in cans).
In the film The Big Lebowski the character of the Stranger asks a bartender "D'ya have a good sarsaparilla?" to which the bartender replies "Sioux City Sarsaparilla?" The Stranger then says "Yeah, that's a good one".