I have a machine running Debian Linux (stable), but I'd also like a Debian unstable box to use as a sandbox to test and develop. Is there an easy way to create a virtualized Debian unstable machine on my Debian stable box?
If you want the alternate distribution as a development environment, and you don't need to run services (or only a few selected ones) or a different kernel in the unstable installation, put it in a chroot. See this guide (replace 64-bit/32-bit by stable/unstable).
At the other extreme, if you want a completely separate installation, the easiest way is to fire up a full-fledged virtual machine and install Debian unstable there. VirtualBox is easy to set up; VMware and KVM are reasonable alternatives.
There are other Linux-on-Linux virtualization technologies that provide better performance (less RAM usage, in particular) at the expense of ease of installation and the requirement of running Linux on Linux. Go for this approach only if you need to run all the normal services in the unstable installation, and you can't afford the RAM requirement of fully independent virtualization.
There are plenty of virtualization software for Linux. My personal favorite is VirtualBox since it' easy to use. Just install it, run it, create an empty machine and install unstable on the virtual machine.
You can find more detailed instructions on the VirtualBox site or just ask.