Red Hat 9 (aka RH9) is an old version of the Red Hat operating system, created and distributed under a free licence by Red Hat Software Inc.
After RH9, Red Hat decided to change their business model, and start releasing their operating system as "Red Hat Enterprise Linux" (RHEL), charging for both the software and its support. RHEL5.1 is the latest version currently offered by Red Hat.
You can still re-distribute the software for free, but you have to pay for the original download, and for every installation that you want to get support for.
If you want a more open distribution, then you could try looking at Fedora (which is an open-source, freely distributed operating system based on Red Hat for which commercial support is not available), Debian (highly stable and rarely changes), or Ubuntu (based on Debian, has more frequent updates and commercial support available through its makers, Canonical).
There are literally thousands of Linux distributions, but I think they are the main server ones. http://distro-watch.org/
Hope that helps,
—Robert J Lee