They've been in the public domain for years (and consequently in dozens of DVD compliations) but Warner Bros. has uploaded the classic Superman cartoons from Fleischer Studios.
In my totally unprofessional opinion, these are among the very best early animations of ol' Supes, bringing together the best of the early rotoscope animation style with the classic wonder of the Golden Age. There's no time-defying Kryptonian to wind back the clock by circling the Earth here--just a guy leaping tall buildings in a single bound (okay, he does fly a bit). Oh, and tearing the shiznit out of some robots!
I've stuck with a properly themed sci-fi episode, Mechanical Monsters, which later went on to inspire some even BIGGER automatons in movies like the Iron Giant and Sky Captain: World of Tomorrow.
Check out the full list of the Fleischer toons at Cartoon Brew.
Incidently, Hydra Miniatures has some excellent "warbots" that look like good stand-ins for Iron-Sky-Fleisher-bots!