I am aware of the Hardened Linux from Scratch project which is a project that provides you with step-by-step instructions for building your own customized and hardened Linux system entirely from source. I would like to know what is the equivalent in BSD?
As Richard said OpenBSD is definitely worth a go, it is my #1 choice for everything that is dedicated for firewalls and gateways. For other services I tend to stick to FreeBSD although there is no obvious reason for it just a personal preference.
But I would like to point out that the from 'scratch part' concept if you want to do more secure hosting of a service can be much better done using Jails. In essence you create a limited FreeBSD environment on an a full FreeBSD install. In that limited environment you only copy/link those binaries and files that the service requires to run.
Because the hosted service has no access to any other files/binaries, all the potential security flaws in those things aren't open to exploit. If by chance your application gets 'rooted' it will not go beyond the boundaries of the jail.
See it like a sandbox on steroids with neglectable performance penalties.