Red Hat Enterprise Linux 6.3 (RHEL 6.3) was originally released on the 21st of June, but the CentOS team is available with their updated distribution based upon these Enterprise Linux 6.3 packages.
As with upstream RHEL 6.3, CentOS 6.3 key features include switching to LibreOffice as the office/productivity suite on desktop installations, the Matahari API for operating system management has been deprecated, and there's new tools for moving physical and virtual machines into Virtual KVM machine instances. These new tools from Red Hat are virt-p2v and virt-v2v for physical-to-virtual and virtual-to-virtual migration, respectively. Plus there's various other improvements that have come about in recent months since the release of CentOS 6.2 last December.
The CentOS i386/x86_64 release announcement can be found on the CentOS mailing list while the release notes provide greater detail on this free re-spin of RHEL.
Meanwhile, Red Hat is also busy working on Red Hat Enterprise Linux 7.0.
As far as some of the other RHEL-based distributions out there, Oracle has already released Oracle Linux 6.3 while Scientific Linux 6.3 is still a few weeks out.