You can use Qemu to boot a CD ISO image you downloaded before you even burn it. This could be useful for just checking it out before you decide to spend a CD for burning it or to take screenshots you otherwise wouldn't be able to from the real bootup.
Here's a simple command to do it:
qemu -cdrom cdimage.iso
You can even install a CD image on a special virtual disk you can create on qemu. This disk is actually not a disk, of course, but a disk image which contains free space that can be used within qemu emulation.