Bonjour is Apple's implementation of the Zeroconf service discovery protocol. mDNSResponder is the open source software from Apple that implements Bonjour in both Mac OS X and Darwin.
To manually announce services on the local network, you can use the mDNS command.
# Announce HTTP server
mDNS -R . _http._tcp . 80
# Announce AFP server
mDNS -R . _afpovertcp._tcp. 548
More service types are defined at http://www.dns-sd.org/ServiceTypes.html
Servers on Darwin systems are usually launched through launchd. You can use the Bonjour key in LaunchDaemons plist files to have launchd automatically announce services.
To browse announced services on the Mac, you can use Bonjour Browser.