Damn Small Linux is really a damn small linux distribution till date. Amazingly the entire ISO image could fit in a pen drive or in a small compact disk because the total size of the ISO is only 50 mb. More here
It acts as live CD meaning that the OS can boot from any system withoutinstalling in the hard drive. DSL can even be operated from within Windows also. So you cancarry the entire OS with you and it can fit in your wallet also and you can work on anysystem. DSL not only works as a complete desktop suite but it can be used as SSH or FTP orHTTP(Web) server and that also right off of the live CD.