Simplestack is an open source project that provides a generic API for managing virtual machines across different hypervisors like Xen, VMware, KVM, and HyperV. It aims to solve the problem of hypervisor lock-in by starting with XenAPI and allowing applications to work with any hypervisor through a common interface. Simplestack exposes RESTful APIs for machine creation, editing, power operations and other features while keeping the implementation stateless and atomic friendly for easy deployment in real-world testing.