Google will take you to "Redhat Download" in a sec.
Free to download, but not free to use :
No updates, no software packages can be installed, without an account.
I.e. do not use Redhat, if you don't want to pay
CentOS 5.5 and ScientificLinux SL55 are exact copies of Redhat,
just with no Redhat logo : Both are free to use and updated with the same
security patches as Redhat.