The software is part of OpenBSD. Other options and supported systems are listed here.
This project didn't release any stand-alone binary distributions yet. Please get and install OpenBSD that ships with relayd and httpd or download the source code.
Experimental branches and Git mirrors are also found on GitHub. Please note that the primary development happens in CVS and that the Git mirrors are usually out-of-date.
relayd is primarily developed for OpenBSD but can be ported to other operating systems. A known port that is actively maintained is available for FreeBSD. Please let us know if you're working on a port for another system.