Although Superman is as far from the public domain as a fictional character is likely to ever be (except for Mickey Mouse perhaps) the first cartoon serial from 1941-1942 is however, as are a lot of the work by Fleischer Studios (like the Gulliver’s Travels feature I already posted). This series is perhaps most well remembered nowadays as the main inspiration for the retro style of Batman: The Animated Series in the early 90’s.
Superman (1941-1942 animated serial)
https://www.youtube.com/watch?v=pgLMH0NGvhg - from the 8thManDVD.com™ Cartoon Channel YouTube channel (2 hours 26 mins long - 720p)
A compilation of all the Superman cartoons produced between 1941 and 1942.
Directed by Dave Fleischer, Seymour Kneitel, Isadore Sparber & Dan Gordon.
Cast: Bud Collyer & Joan Alexander.
The Mad Scientist)
The Mechanical Monsters
Billion Dollar Limited
The Arctic Giant
The Magnetic Telescope
Terror on the Midway
The Mummy Strikes
The Underground World
The series was originally produced by Fleischer Studios, which completed the initial short and eight further cartoons in 1941 and 1942. Production was resumed in May 1942 by Famous Studios, a successor company to Fleischer, which produced eight more cartoons in 1942 and 1943. Superman was the final animated series initiated by Fleischer Studios, before Famous Studios officially took over production.
IMDb link to first episode