LXD is an open source system container manager, developed by the team behind LXC, written in Go and that's been around for over 5 years now. It's widely used both on servers, running the backend of services such as Travis-CI and on everyday devices like Chromebooks. Containers are created from images with prebuilt images available for most Linux distributions.
Multiple hosts can easily be clustered together to form one large virtual host, exposing the exact same API as a single host would. Storage pools and networks can also be created and managed through LXD and resources can be segmented into projects.
With the addition of virtual machine support (through qemu), it is now possible to manage a mixed deployment of virtual machines and containers, sharing the same configuration, storage and networks. With the use of our built-in agent, the exact same operations that are normally possible against a container also become possible against virtual machines, including executing commands, transferring files, ...
Existing API clients for LXD can also now drive both containers and virtual machines without any API changes required.