simple/basic rust CAS:
https://github.com/p-e-w/savage

rust PEG parser library, with nice error recovery (aka, human-readable parser errors):
https://github.com/zesterer/chumsky
https://github.com/zesterer/ariadne

e-graphs:
https://egraphs-good.github.io/
https://www.philipzucker.com/egraph-1/
https://www.philipzucker.com/egraph-2/
https://blog.sigplan.org/2021/04/06/equality-saturation-with-egg/

Julia CAS stuff:
https://github.com/JuliaSymbolics/Metatheory.jl
https://github.com/JuliaSymbolics/TermInterface.jl