Use Windows 8 Hyper-V to create an app sandbox or test machine
Virtualization makes it possible to sandbox apps -- or even whole variant installations of your main OS -- for the sake of testing or evaluation. If you have a program you want to try out but are leery of putting it on a production system, stick it in a Client Hyper-V VM. One of the finest analogies I've ever heard for this was that it's like a roll of paper towels: When you're done using that app, you just wipe down the VM or roll it back to a snapshot. You don't even have to uninstall the program in question.
Snapshotting lets you save the state of the system image, so you can revert to it after you're done testing, in much the same way a backup of a system lets you roll back to the point when the backup was made. That said, snapshots of Client Hyper-V VMs are by default stored in the same directory as the VHD file for the VM, so taking too many snapshots can crowd out space that might be needed by the VHD as more items are added.
This matters twice as much if you use that disk for storing other things. You don't want to take one snapshot after another, then have other programs start bombing because they don't have storage space left.
Another thing to keep in mind about snapshots: Snapshots of a VM reduce disk performance for that VM. Each additional snapshot made of a VM creates that much more of a performance bottleneck, since both the snapshot(s) and the underlying VHD have to be processed. If you're using snapshots as a way to keep a clean test machine, use only as many as you absolutely have to. This rule applies for most any use of Client Hyper-V where you're snapshotting, too.
Finally, using Client Hyper-V as a test platform doesn't change the rules about licensing. You'll still need to have a separate license for the instance of Windows you're running. An MSDN or TechNet subscription is one good way to deal with this, since you can use an OS license from the pool given to you for a testing/evaluation install. For your yearly membership fee, you get tons of other software you can evaluate in the same way.