I think AsmGrid has a great overview of X86 and AArch64 instructions:

  - https://asmjit.com/asmgrid/

For x86 encodings, there’s also http://ref.x86asm.net/index.html and of course the venerable https://sandpile.org/.

Felix Cloutier's page has always been my go-to

I haven’t seen this site before. Thanks for sharing it.

wow, that's great. thanks for sharing!