HAProxy is not part of CentOS, but it used to be part of the Fedora EPEL repository. However since RHEL6.4 HAProxy is part of Red Hat's Load Balancer product/addon and EPEL's policy to not conflict with Red Hat's offerings resulted in HAProxy to be removed from Fedora's EPEL repository.
Since HAProxy is part of RHEL's released product (albeit as an addon and a Technology Preview) it means CentOS does include it in their normal CentOS-6.4 offering (much like it includes other addon products, e.g. High-Availability). This means CentOS includes it in its main repository.
Beware that, while it may not be obvious to a lot of CentOS users, haproxy is a Technology Preview, which means that it is not (yet) supported by Red Hat, either because Red Hat does not feel comfortable supporting it already, or because they expect another major/minor release update, or the roadmap for the product is still changing (new functionality, known bugs). Whatever the reason the lack of support means there are reasons for Red Hat to assume the support cannot be guaranteed, which is something a CentOS user should be aware of as the CentOS project does not specifically mention this.