I'm quite a fan of Charles Ives' atonal work. I think I heard somewhere that he was developing an atonal system at the same time as Schoenberg without knowing about him, but I can't find anything to validate that so I could be wrong.
There's also Krzystof Penderecki, who did several soundtracks that you may have heard.
Edgard Varese is the man
There's also Stravinsky, Debussy, and John Cage. If I were you I would check out all of Schoenberg's work because it's fantastic, especially this early piece.
The pieces I posted may not be strictly atonal, but they give you an idea of what the artists are about

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