The Project Akita OS (Orange OS) was started with the intent to design a lightweight operating system. It contains a small kernel designed for those interested in the low-level, practical aspects of building an x86 system. The host development environment is designed to work around the Bochs x86 emulator, so that you can easily test changes to the kernel.
blueflops is a two-floppy Linux distribution with a graphical Web browser (links) and an IRC client (rhapsody). The kernel has most of the Ethernet drivers compiled in as modules (including the PCMCIA ones) and PPP support (for dialup connections). The browser is linked with OpenSSL. The scripts are all accessible by a setup script and have a nice dialog frontend.
alcolix is a minimal Linux rescue distribution with the goals of being small, compatible, and very usable. It has a cozy shell and a multitude of partition rescue/editing tools, all based on up-to-date releases (e.g., 2.4.x kernel with USB support). It uses cpio.bz2 data disks and has a full GRUB bootloader, memtest86, and more.
Salvare (from the Latin "to rescue") is a small Linux distribution designed for small, credit-card sized CDs which typically hold around 34MB. More Linux than tomsrtbt but less than Knoppix, it aims to provide a useful workstation as well as a rescue disk. In addition to providing a rescue environment similart to tomsrtbt, it adds OpenSSH, a graphical and text mode Web browser (Links), security tools such as chkrootkit, nmap, and tcpdump, the ability to apt-get install additional software from Debian into tmpfs, the ability to run entirely from RAM, and much more. It's also customizable using a simple source-code based package management system.
The STUBS Toolchain and Utility Build Suite is a set of scripts which, together with a set of pre-written configuration files, builds one or more software packages in sequence. STUBS is designed to work in very minimal environments, including those without "make", and URLs are included so source and patches can be downloaded as necessary. Configuration files and scripts are provided which create boot media for Franki/Earlgrey Linux (one of several example busybox- and uClibc-based Linux environments) and the intention is that STUBS should be able to rebuild such an environment from within itself.
MBCD (MultiBootCD) is a shell script to make a customized CD-ROM that can boot any kind and any number of image files. Currently, 4 types of images are supported: floppy images (1.2M, 1.44M, or 2.88M), Knoppix-like images, kernel-binary images (e.g. memtest86), and the Windows XP Recovery Console.