Now, it's certainly true that you can install the Android SDK and use its included emulator to run various versions of the OS, but that's a LOT of overhead; I don't need to dive THAT deeply into Android internals, and--to be honest--the performance of the emulator isn't all that great. I recently discovered the Android-x86 project, which has been going strong since 2009 to bring Android to the x86 platform. I installed Android-x86 on an old netbook and started playing with it, and then I realized...why not run it in a VM?
Enter Oracle VirtualBox.
This free virtualization package is available for Windows, OS X, Linux and Solaris; I'm currently using it on my Windows 7 laptop and several of my Ubuntu Linux machines, so I tossed an Android-x86 ISO into a new VM and went to work. Ten minutes later, I had this:
|Android-x86 5.1RC1 running under VirtualBox on Windows 7|
If you need to work with several versions of Android, Android-x86 can help you there as well; you can download ISOs of Lollipop, KitKat, Jellybean and Ice Cream Sandwich and install them to their own VMs. You can also share/copy VirtualBox VMs across multiple platforms (for example, I moved an Android-x86 VM from one of my Linux systems to my Windows 7 system with no problems). While I haven't done it myself, I'm told that some enterprise admins have registered their Android-x86 VM with their mobile device management (MDM) products of choice for use in testing/prototyping...
(NOTE: You can install Android-x86 to a bootable USB stick, if you so desire; here are the details.)
(NOTE #2: If you have a touchscreen laptop sitting around, give it a shot! Here's a video of Android-x86 4.4.2 (KitKat) running on a Lenovo Y50.)
So, whether you're testing, developing, or just want to play around with Android without buying a device or jailbreaking your personal stuff, take a look at Android-x86 and VirtualBox; they make a good pair.