OS X as an NFS server

Everything you need for this is built right in.

OS X Tiger

Here's the output of nidump -r /exports /

  "name" = ( "exports" );
      "clients" = ( "" );
      "name" = ( "/usr/local/soekris" );
      "opts" = ( "ro" );
Now reboot and OS X will automatically start rpc and nfsd when it detects the exports netinfo setup.

OS X Leopard

It would appear that OS X Leopard has removed the "exports" section of netinfo and relies exclusively on the /etc/exports file and the 'nfsd' command.
So, simply create an /etc/exports following the instructions in exports(5) ("man exports"), type "nfsd enable", and verify with 'showmount -e'.