Here is a great interview with Bob Wozniak from Quaker Boy Game Calls. Quaker Boy Manufactures a wide range of Howlers, Squallers and Squealer coyote calls- as a matter of fact, if they don't make it you probably don't need it. New for 2011 Quaker Boy has introduced a family pack of calls, "The Three Dog Night". The Three Dog Night call pack includes the Alpha Howler, a female call that is a bit higher pitched and a pup which is higher pitched then the female.
Another popular call for extreme cold weather hunters is the Squeezin Screamer which uses totally ambient air so you never lock up or freeze up like a mouth blown call can in sub zero temps.
More Info On Calling Coyotes: