There are a few other virtual labs that I use for this book and other purposes and GNS3 is one of them:

As discussed, GNS3 is what a lot of us used to study for test and practice for labs. It has really grown up from the early days into a viable commercial product. Cisco-made tools, VIRL, DevNet, and dCloud only contain Cisco technologies. They provide ways to communicate to the interfaces outside of the lab; however, they are not as easy as just having other vendor's virtualized appliances living directly into the simulation environment. GNS3 is vendor neutral and can include other vendor's virtualized platform directly either by making an clone of the image (such as Arista vEOS), or by directly launch other images via other hypervisors (such as Juniper Olive emulation). GNS3 does not have the the breadth and depth of the Cisco VIRL project, but since they can run different variation Cisco technologies, I use it many times when I need to incorporate other vendor technologies into the lab along with some Cisco images.

There are also other vendor virtualized platforms such as Arista vEOS (https://eos.arista.com/tag/veos/), Juniper vMX (http://www.juniper.net/us/en/products-services/routing/mx-series/vmx/), and vSRX (http://www.juniper.net/us/en/products-services/security/srx-series/vsrx/) that you can use as standalone virtual appliance. They are great complementary tools for testing platform specific features, such as the differences between the API versions. Many of them are offered as paid products on cloud provider market places and are offered the identical feature as their physical counterpart.