Debian From Scratch is really two related components in one. First, it's a live bootable CD image that is designed to provide a full-featured kernel and rescue environment. It includes filesystem tools, editors, C compilers, Python, Perl, and everything you need to manually install Debian on a new machine. On ix86 machines, Grub is used to boot directly from the CD. There is also a program that is used to create the DFS CD images. It is highly configurable and can be used to create other custom bootable CD or DVD images. You can include whatever kernel or packages you want.
xtee (cross-tee/expanded tee) is a program for building complex pipelines. It resembles the tee command, except that instead of copying stdin to stdout, it copies an input file to stdout and stdin to an output file. You can use xtee for building things like a bidirectional HTTP filter (using netcat and sed).