Debootstrap is a project mainly written in Shell, it's free.
debootstrap (debian) for koji
See the manpage for (some) documentation.
You can run debootstrap from its source tree without installing it. This can be useful if you want a quick way to make a Debian chroot on another system, or if you are testing modifications to debootstrap.
First, get the source.
Either by using subversion: svn checkout svn://svn.debian.org/d-i/trunk/packages/debootstrap
Or by visiting http://packages.debian.org/source/sid/debootstrap and downloading the tar.gz file
Then as root, in the debootstrap source directory:
make devices.tar.gz
export DEBOOTSTRAP_DIR=pwd
debootstrap sid sid
Cross-strap support - so you can bootstrap a filesystem to the point where it will successfully boot, and finish installing itself without having to be running the target architecture or OS yourself. This means you should be able to run
debootstrap --arch powerpc sarge ./sarge-ppc-chroot ...
on an i386 system, boot a powerpc box with sarge-ppc-chroot as its root files system, and have it "work". The cross-hurd package does something similar, and should be replaced by this feature.
There should be some (better) way of telling debootstrap what "base" packages you want to install -- this varies between making a chroot, doing an install, and doing a buildd. Also, some installs want different base packages (to setup networking, or kernels, eg)
If there's a problem with debootstrap that you need fixed, feel free to do an NMU to fix it. Usual rules: try not to break anything, and mail the patch to the BTS. Don't worry about asking first though.
However, note that debootstrap is now team maintained. Anyone in d-i can do a release without the bother of a NMU.