Taken straight from ibiblio.org's Linux distro site:
BasicLinux
BasicLinux is a multipurpose mini-Linux that boots from HD, FD or CD-ROM. The 2mb package provides the usual rescue/repair tools, but it can also dial an ISP and browse the net, or act as a router/firewall. BasicLinux uses a newbie-friendly shell and editor, and it would make a good introductory Linux for a DOS dinosaur.
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Coyote
Coyote Linux is a single floppy distribution of Linux that is derived from the Linux Router Project (LRP)
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Dualix
GNU Dualix is a mini-distribution of Linux. It is intended for use as a portable network client, but includes much more than just network programs - look below for a partial list. Not that this is not a recue system.
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Frazierwall
Frazierwall Linux is a single floppy distribution of a NAT firewall that is based on the Linux Router Project. It is designed for use with Ethernet based internet connections (such as cable modems or xDSL lines). FrazierWall allows you to share a connection with several other computers on a LAN. The FrazierWall is remotely manageable via telnet and with a web browser.
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Giotto
Giotto is a floppy Linux: a bootable floppy disk that comes with the necessary parts of the Linux operation (
sic) system. It can be installed also on a harddisk and runs completely out of the system's RAM, generally no hard disks are required.
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Linux-router
A networking-centric micro-distribution of Linux. LRP is small enough to fit on a single 1.44Mb floppy disk, and makes building and maintaining routers, access servers, thin servers, thin clients, network appliances, and typically embedded systems next to trivial.
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Not sure if this one qualifies:
NFSRoot-Linux
Using this system we get a light, usable linux console on our machine. On local hard-drive (or optionally floppy disk) it takes only about 400k (kernel and loadlin), while all the root is on an nfs server.
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Peewee Linux
A Linux distribution for embedded applications and single floppy systems. Features an ncurses graphical configuration utility.
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Small-linux
Small Linux is a distribution which fits on a floppy and can boot in less than 2 megs of RAM. It is intended for users who need a Linux Kernel that meets small memory requirements. Small Linux has been used (console based) on a 386 laptop with 2 meg of ram and 40 meg harddrive.
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Trinux
Trinux is a portable Linux distribution that boots from a single floppy disk, loads its packages from a FAT/Ext2 partition, floppy disks, or HTTP/FTP, and runs entirely in RAM.
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I have not tried any of these, so I cannot vouch for any of them - but they're all Linux, so they can't be that bad!