[ruggeri]

Time to betray some serious virtualization ignorance; when do people use QEMU outside a datacenter (presumably for mapping multiple virtual machines to one physical machine)? How often does one need to run binaries compiled for another machine architecture? And setting aside banal scenarios like running Word on Windows inside Linux, when is a guest OS useful?

[aliguori]

The Android SDK is based on QEMU. This is what lets you develop and test ARM binaries for Android on your x86 PC.

There are many examples of embedded development environments like this.

[JoshTriplett]

I run KVM quite regularly on my laptop. I use it to test the results of Linux kernel hacking, to test bootable Linux images of various types (installers, live images, demos), and to test BITS (http://biosbits.org/) without having to run it on dedicated hardware (at least, the bits that don't specifically test that dedicated hardware).

[dignan]

I use VMs to test for bugs in my code on other platforms, and to test new releases of various Linux distributions.

[guard-of-terra]

Let's say you have a hard drive with DOS and Stacker and you need to copy files from it.

Or you have a disk image and need to copy files from that, and for some reason you can't mount it (Windows or virtualized linux without loopback block devices)

qemu does wonders to disk images.