Johnny Snow is an ice-themed superhero who emails threateningly to Dr. Horrible, whom he thinks of as his nemesis. Horrible seems to suggest that Johnny Snow, who is never actually seen, arms himself with an ice beam and wears a parka (a reference to a DC comics villain Captain Cold).
Johnny Snow is referenced only twice; this occurs immediately before and during the Freeze Ray song.
He possibly alludes to a character from Buffy the Vampire Slayer who had a freeze ray and was named Jonathan Levinson.
There is actually a game where the main character wants to make it snow in San Fransisco. Johnny Moseley AKA Johnny "Snow"