Ever wanted to test the new K/X/Ubuntu 7.04 Feisty, but don't have a spare partition, and don't want to jeopardize your production environment by resizing partitions, dist-updating, or tinkering with its bootloader? Now you can leave your existing Linux distro untouched, while being able to use Ubuntu 7.04 Feisty in a full-fledged install on a loopmounted partition, no partitioning required!

Basically, it downloads a K/X/Ubuntu alternate i386 iso, creates loopmounted disk images so that they can be installed there, and adds an entry into /boot/grub/menu.lst which starts the d-i installer with that iso, and installs it into the loopmounted disk images. What thus results is a dual-boot system, in which K/X/Ubuntu are installed in loopmounted disk images in the folder /wubi/ on the filesystem, so that they can be installed without requiring any repartitioning, and they are booted using the system's GRUB, with the last menu entry, "Ubuntu", being added by the script to boot the loopmounted disk images.

More Here.