Recs.
Updated
Puppy Linux is a distribution designed for low end machines, and also designed to be ran from external media (USB drive, CD/DVD, etc.)
It is lightweight and easy to use, simple to setup, and good if you need a portable Linux distribution on the go.
Specs
Pros
Pro Boots fastest
It boots fastest among all on this old single core laptop (- about 3 times - even when compared to tiny/lightweight distros). Therefore you can also use it as an emergency tool to quickly start your pc from usb etc. and at least save your files & folders on disk to another safe place...
Pro Will run from almost any install media
The recommend media size for puppy Linux is at least 400MB, which means it will fit on MiniCDs, CDs, DVDs, USB drives, and External Hard drives.
It is better to use a USB drive with at least 4GB of storage space for optimal usage. The operating system itself will take up around 400MB, and you will have about 3.5GB of space for your files.
Pro Different distro based versions available
Puppy Linux offers a few different releases.
The first is an Ubuntu based release called Tahrpup, by using this version the user is able to take advantage of all software and support from Ubuntu.
The second version is called Slacko, which is built upon the Slackware binaries.
The third is called Wary and it is built to support older hardware than the rest.
And the fourth is called Quirky, which is used as a base to explore new ideas.
Pro Designed to run as a live USB/CD
A portable version, that can be carried on a thumb drive without requiring installation, weighs less than 100MB; a Live CD - less than 150MB. You can even save your settings and files for Puppy on the external device/media.
Running off a live CD also has security benefits, as your system will reset to the known config after each boot.