Mountebank is an open-source tool designed for testing and developing applications through service virtualization. It allows developers to create mock servers that mimic real system services, enabling efficient testing without relying on the actual services being available. With support for multiple protocols such as HTTP, HTTPS, TCP, and SMTP, Mountebank facilitates sophisticated testing scenarios and helps in identifying issues early in the development process. It is particularly useful in distributed systems where integrating and testing against all dependencies can be challenging.