Bluestacks.com is a free Android OS emulation platform that can be downloaded on Windows 10 and macOS software. The software's basic features are free to download and use. As of 2019, Bluestacks.com reports it's able to run over 1.5 million Android apps. App Player features mouse, keyboard, and external touchpad controls. Great for sandboxing malware and performing forensics.
Network 169 offers several routing troubleshooting challenges and configure 'how-to's' using GNS3 or VIRL platforms. Also offering great blog discussions around each platform differences.
Mininet is a great, quick way to spawn virtual networks for testing. The lightweight Ubuntu-based OS will create virtual servers and devices capable of communicating with the built-in SDN controller.
TechNet Virtual Labs
Microsoft's TechNet Virtual Labs is a powerful online environment that enables testing of new Microsoft products and technologies. Virtual Labs will walk you through, step-by-step, navigating multiple VM's to accomplish a complex technical task. It also explains each step using clear, simple terminology to help you learn quickly and efficiently.