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.
Microsoft* Windows 95 was just released (again). This time in Electron and available on GitHub for both Windows and macOS.
*not approved or associated with Microsoft
A great thing to have onhand is Ubuntu Desktop OS on a USB stick. Especially since USB sticks are priced at an all time low. For example, on a corporate laptop equipped with full-disk encryption, offline network policy enforcement, user access inside Windows, and a highly customized desktop experience removing ability to right-click, view options, or see the Control Panel. Fortunately, promptly hitting F10 at boot time allows access into boot options, select USB, test Ubuntu, enable Multi-verse libraries, enable VLC Player and watch movies. Bypassing all the corporate security and allowing us to actually use the laptop hardware. You can also use it to recover files from a damaged Windows machine.
GNS3 is a free, open-source tool for testing network configurations, proposed changes, or preparing for Cisco certification. GNS3 uses a combination of technologies to produce a testing environment - VirtualBox, Dynamips, and Qemu. The difficult part is acquiring IOS image files as Cisco limits downloads now to SmartNet contracts, e.g. if you own the device you can access the respective IOS or NX-OS files. GNS uses the files to virtualize the network devices.
Citrix Provisioning Server
This post links to a fantastic YouTube video of "Ardence" in action. In action meaning that 1 server is streaming 1 OS image, real-time to 500 desktops then rebooting into a different OS. Since this was so sweet, Citrix bought Ardence and renamed it Citrix Provisioning Server. Since then I've had the pleasure of creating some really cool PVS servers and trained IT on creating images and image management.
My first experience with Cisco UCS chassis and blade servers was a memorable one. Virtualization is new to most Cisco engineers it seems because managing the UCS firmware is almost always performed incorrectly and is the cause for various communication, application, overall server performance issues. UCS produces a central status report showing green and red lights regarding system health. This can be used to quickly identify firmware mismatches which is the most common cause of PSOD (purple screen of death) hit to the hypervisor.