QuickcCheck-type tools (generators for tests that know about the edge cases of a domain - e. g. for the domain of numbers considering things like 0, the infinities, various almost-and-just-over powers of two, NaN and mantissas for floats, etc.):

* QuickCheck: https://hackage.haskell.org/package/QuickCheck

* Hypothesis: https://hypothesis.readthedocs.io/en/latest/

* JUnit QuickCheck: https://github.com/pholser/junit-quickcheck

Fuzz testing tools (tools which mutate the inputs to a program in order to find interesting / failing states in that program). Generally paired with code coverage:

* American Fuzzy Lop (AFL): https://github.com/google/AFL

* JQF: https://github.com/rohanpadhye/JQF

Mutation / Fault based test tools (review your existing unit coverage and try to introduce changes to your _production_ code that none of your tests catch)

* PITest: https://pitest.org/

Hypothesis for Python.

Schemathesis builds on Hypothesis and generates tests from OpenAPI specs.

And from GraphQL APIs as well :)