Linux in under 50MB

Damn Small Linux is a distro created to be, well, small.

Some of what it can do:

• Boot from a business card CD as a live linux distribution (LiveCD)
• Boot from a USB pen drive
• Boot from within a host operating system (that's right, it can run *inside* Windows)
• Run very nicely from an IDE Compact Flash drive via a method we call "frugal install"
• Transform into a Debian OS with a traditional hard drive install
• Run light enough to power a 486DX with 16MB of Ram
• Run fully in RAM with as little as 128MB (you will be amazed at how fast your computer can be!)
• Modularly grow -- DSL is highly extendable without the need to customize

Link:
http://www.damnsmalllinux.org/