The Simulation Argument is certainly quite an interesting one, since it was invented by an atheist (Nick Bostrom), and as far as I can tell is only taken remotely seriously by other atheists. Many of them (including me) think it is a rather better argument for some sort of "god" or "gods" than anything theists themselves ever came up with.
For other interesting quasi-theistic arguments invented by atheists, you might want to consider Tegmark's Level 4 multiverse. Since any "god" which is logically possible can be represented by some sort of mathematical structure, it exists somewhere within the Level 4 multiverse. David Lewis' modal realism has a similar feature.
All these arguments tend to produce massively polytheistic rather than monotheistic conclusions (and also they imply that Santa, the Tooth Fairy, Harry Potter and Captain Kirk exist somewhere or in some simulation or other).
If you want a fun monotheistic argument invented by atheists, try this one, which was published by Robert Meyer and attributed to Hilary Putnam. It's a clever use of the Axiom of Choice and Zorn's Lemma.
On reflection, the fact that an atheist would be able to come up with an argument for a god that's more persuasive to atheists is unsurprising, especially when you consider the fact that most religious people don't become religious via being persuaded by arguments. It's definitely still amusing, though.
I'm definitely aware of Tegmark's theory, though I admit I hadn't considered it as an argument for any kind of theism. That seems like an awfully parochial and boring application of the ultimate ensemble, although you're right that it can have that sort of a...
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