Owl (Openwall GNU/*/Linux) is a small security-enhanced Linux distribution for servers. Owl also makes a good base system for customized virtual machine images and embedded systems, and Owl live CDs with remote SSH access are good for recovering or installing systems (whether with Owl or not). A single Owl CD includes the full live system, installable packages, the installer program, as well as full source code and the build environment capable of rebuilding the entire system from source. Owl supports multiple architectures (x86, x86-64, SPARC, and Alpha) and offers some compatibility for packages developed for other Linux distributions. The primary approaches to security are proactive source code review, privilege reduction, privilege separation, careful selection of third-party software, safe defaults, and "hardening" to reduce the likelihood of successful exploitation of security flaws.
Chakra Linux is a Linux distribution that combines the simplicity of Arch Linux with KDE. It is fast, user-friendly, and extremely powerful. It can be used as a live CD or installed to hard disk. Chakra is currently under heavy and active development. It features a graphical installer and automatic hardware configuration. Chakra provides a modular and tweaked package set of the KDE Software Compilation with a lot of useful additions. It features the concept of half-rolling releases and freshly cooked packages and bundles. It is designed for people who really want to learn something about Linux or don't want to deal with administrative overhead.
BG-Rescue Linux is a Busybox and uClibc-based rescue system with a 2.4 series Linux kernel. It supports full read/write support for NTFS using ntfs-3g. It is loaded from two floppy disks, a bootable CD-Rom, or a USB Stick. The system runs entirely in RAM. It supports a wide range of hardware (including SATA and DM/LVM2) and filesystems (including ext2/3, hfs/hfs+, and xfs) and can serve as a full backup/restore system for MS Windows systems which use FAT12/16/32 or NTFS. It also includes many utilities, including strong compresstion tools like lrzip. Automatic loading of the antivirus-program F-Prot from cdrom or usbstick is supported.
MikeOS is an operating system for x86 PCs, written in assembly language. It is a learning tool to show how simple OSes work, with well-commented code and extensive documentation. It features a text-mode dialog-driven user interface, a file manager and machine code monitor, and over 60 system calls for use by third-party programs.
Tin Hat is a Linux distribution derived from hardened Gentoo. It aims to provide a very secure, stable, and fast desktop environment that lives purely in RAM. Tin Hat boots from CD, or optionally USB pen drive, but it is not a LiveCD in that it does not mount any file system from the boot device. Rather, Tin Hat employs a massive squashfs image which expands into tmpfs upon booting. This makes for long boot times, but remarkable speeds during human-computer interaction.
Slax is a modern, small, and fast Linux operating system with a modular approach and outstanding design. Despite its small size, which is below 200 megabytes, Slax provides a wide collection of pre-installed software for daily use, including a well organized graphical user interface (KDE4) and useful recovery tools for system administrators. The modular approach gives you the ability to include any other software in Slax easily, by downloading a module with the software, without having to install or configure it.
Arch Linux is an i686-optimized Linux distribution. It is lightweight and contains the latest stable versions of software. Packages are in .tar.gz format and are tracked by a package manager that is designed to allow easy package upgrades. Arch is quite streamlined compared to some other distributions. Things that are relatively unused are not kept (info pages, for example). A default Arch install leaves you with a solid base; from there, you can add packages to create the custom installation you're looking for. Arch has a package build system that allows you to easily create your own packages, which makes it very easy to rebuild a package with your own custom configuration. Arch also aims to use the newer features available to Linux users, such as reiserfs/ext3 and devfs.