LFS News : LFS 6.4 released! This is Linux From Scratch, one of the more fun linux releases out there. It's not particularly useful once installed, being a very vanilla linux with no GUI unless you add one. No, the beauty of this linux is that you literally compile the entire thing from source - all of it.
You need a host environment, so they provide a live CD, and inside that you compile all of the code from the command line, no scripts (at least, not when I did it), you type every command and all parameters. You even get to type parts of the source in the beginning.
It's an incredibly good learning experience, and I may just have to do it again.