CentOS would typically be the closest to RHEL, but CentOS 6 has not been released, yet. The timeline for them to release 6 is bit ambiguous as their forums state they are focusing on updates to CentOS 4 and CentOS 5 first. That being the case, your best bet is most likely Scientific Linux which also builds from RHEL source.
http://www.scientificlinux.org/
EDIT: Sorry, knudfl beat me to it.