TrinityOS is a step-by-step, example-driven HOWTO on building a very functional Linux box with strong security in mind. TrinityOS is well known for its strong packet firewall ruleset, Chrooted and Split DNS (v9 and v8), secured Sendmail (8.x), Linux PPTP, Serial consoles and Reverse TELNET, DHCPd, SSHd, UPSes, system performance tuning, the automated TrinityOS-Security implementation scripts, and much more.
Agnix is a small, educational operating system kernel for i386, supporting 32-bit protected memory mode, paging, hardware switched tasks, memory tests, PCI bus, devices, PCI IRQ routing, RT timers, network protocols. It is available with all the source code and is fully compatible with the Linux kernel API.
gBootRoot makes the construction and development of distributions fun and simple with its Root Methods (Yard) and user-mode-linux test bed. Finish the product with a Boot Method (2-disk compression supported). Normal (non-root) users can make root filesystems and boot disks. It includes the make_debian script to create a testable user-mode-linux base Debian system, add-ons to enhance methods, and an MTD Emulator useful for running distributions made with the jffs/jffs2 filesystem.