A router is a complicated device with many potential functions. The great thing about an OS is that it can do everything a router can do, and more. The downside being that it's not pre-configured, since it has such a vast array of options.
Go ahead and tell us exactly what you intend to do (example: internet gateway and DHCP server) and we'll help you find the right path to getting there.
Gateways and routing:
http://www.freebsd.org/doc/en_US.ISO...k-routing.html (See section 29.2.4, Building a Router)
DHCP server:
http://www.freebsd.org/doc/en_US.ISO...work-dhcp.html
The handbook is an excellent tool.