FreeBSD: Although this is not a Linux distribution, it is a free UNIX-based OS that has much in common with Linux. Many applications made for Linux will run in FreeBSD.
GoboLinux: Linux with a friendly directory structure and no need for a package manager
One of the best known distributions in this category is KNOPPIX.
Ubuntu has a very good LiveCD version
Linux-Stats.org has a client that allows you to add your machine's statistics anonymously. Statistics include kernel version, distro, window manager, file system, hardware, and more.