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.
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.
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.
Tunix is a set of tools used to generate small bootable Linux images. It can be used to learn how to roll your own distribution, or to port embedded systems. The basic release uses busybox and has a kernel with netfilter-enabled modules. A uclibc iptables binary is included, so you can roll your small firewall from a floppy.