The best known as a LiveCD (as opposed to being the most popular with a LiveCD available) is Knoppix.

A LiveCD is a bootable CD that provides a fully functional operating system with no need for installation.

Most of the popular Linux distributions these days have a LiveCD portion, including Ubuntu, PCLinuxOS, and even Gentoo. The most popular known specifically for LiveCD functionality is probably Knoppix.

Yes. All desktop editions of Ubuntu have a LiveCD edition. It is the default and recommended method of installing Ubuntu.

The image downloaded from Ubuntu's website is already a LiveCD. If you want guides on how to customize it, or to create one from an installed system, refer to the links below.