Wilma
Service virtualization: service stub and transparent proxy
Mockae
Open source mock REST API server scripted in Lua
Keploy
Open source tool that turns application traffic into tests and mocks
WireMock
API simulation and HTTP mocking
Mockoon
Local mock REST APIs with desktop app and CLI
Microcks
API mocking and testing for OpenAPI, AsyncAPI, gRPC, GraphQL and SOAP