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.
NASLite is a Network Attached Storage (NAS) server operating system designed to transform a basic computer into a dedicated file server. Utilizing highly optimized versions of Samba, uCLibc, BusyBox, and various other Linux tools, it provides SMB/CIFS, FTP, or NFS filesystem support. It accommodates multiple client OSes: Windows, Mac OS X, and Linux. NASLite offers SMART disk monitoring and large file support, and is incredibly easy to install and administer.
Brazilfw is a mini-distribution designed for setting up network utility services such as Internet connection sharing, firewalling, or wireless access points. The goal is to make it as quick and easy as possible to set up a Linux system with only a minimal amount of Linux knowledge. The main goal of BrazilFW is to continue the development of what was the Coyote Linux floppy firewall system.