Other shows like this?
What are some other shows in terms of mood, animation, and style that are good? I'm thinking stuff along the lines of Aeon Fluxx and The Head.
Try AFRO SAMURAI.
I don't mean to impose, but I am the Ocean.
The two Vampire hunter D movies were quite good, and dark.
Would also suggest checking out the Berserk series, it's one of my favorites and it's quite a dark story mixed with demons and things of that nature like Spawn, though it's set back in the middle ages.
Gantz is a more recent one that is set in modern times with a more dark and twisted story.
Those are Japanese anime though, there just aren't hardly any mature focused animated movies that come out in America like the Spawn series, so you're best bet is the defcent Anime out there.
Only other American released animated movies for a more mature audience I can think of are the older Heavy Metal and Heavy Metal 2000.
I suppose Batman TAS is too obvious? How about Batman Beyond? That series actually gets pretty damn dark and is really good.
Berserk is a good mention, it's pretty cynical and dark. Gargoyles is another great animation show. Then there's great dark Japanese animation shows like Gungrave, Stand Alone Complex and Ergo Proxy.