Puppy Linux is a light weight easy to use Linux distribution that is so weightless that it can run entirely in RAM. But make no mistake, Puppy Linux is a full-fledged operating system.
Puppy Linux first saw the light in 2005. It's developed by Barry Kauler and the open-source community, and because of this, it is well supported. Until recently it didn't use any depositories of other distributions. As of the 5.0 release, it uses the Ubuntu application depository. This means what ever package Ubuntu has can probably also be installed in Puppy Linux. The smallest ISO available is 12Mb. There are a lot of Puppy derivatives also known as Puplets.
This guide to Puppy Linux has links to some great articles on Bright Hub that can assist with the installation and use of this OS.