qemu

TLDR

Boot from image emulating i386 architecture

$ qemu-system-i386 -hda [image_name.img]

Boot from image emulating x64 architecture

$ qemu-system-x86_64 -hda [image_name.img]

Boot QEMU instance with a live ISO image

$ qemu-system-i386 -hda [image_name.img] -cdrom [os_image.iso] -boot d

Specify amount of RAM for instance

$ qemu-system-i386 -m 256 -hda image_name.img -cdrom os-image.iso -boot d

Boot from physical device (e.g. from USB to test bootable medium)

$ qemu-system-i386 -hda [/dev/storage_device]