A self-powered computer in actual credit-card size (~1mm thick) (old.reddit.com)

Slight clarification: It’s battery powered and has no energy generation or harvesting capability.

There's much more information in the linked github repo at https://github.com/krauseler/muxcard

It even includes an eInk display as well as the battery.Insane, but really impressive.

Now just needs an x86 simulator port.[0]

[0] : https://86box.net/

Reminds me of the REX PDA.

https://en.wikipedia.org/wiki/REX_6000