ChallengeOS is an operating system with a native GUI, an interpreter for an object-oriented BASIC dialect, and many more features to make life easier for normal users.
PHP Solutions Live is a bootable Linux distribution which features software for web developers; includes two versions of PHP (running simultaneously), Apache, MySQL, DB Designer, Quanta, phpMyAdmin, Xdebug, and more.
TINY GNU is a small Linux distro designed especially for reusing old computers; has very low demands in terms of hardware capacity, and even a Linux beginner can install it easily.
SoL-diag is a diskless Linux distro that was designed for the rescue and analysis of i686 computers; also useful for improving your Linux skills and benchmarking computers without having to install programs to the hard drive.