There are several Linux Distributions (collections of Linux/GNU-related software) available on the net.
Visit our Distributions category page for an overview of Distros you can find information about on ThinkWiki.
A good starting point for news, comparisons and more detailed information about distributions is distrowatch.com.
You can download CD images (aka isos) of distributions from linuxiso.org, which seems to be unmaintained and out-of-date for quite some time now. A viable alternative is provided by the nice folks of linuxquestions.org, to be found at www.lqiso.org.