EveryDNS.net is a service that provides FREE secondary DNS. Without a donation, you are limited to 20 domain names; with a donation, you are (theoretically) allowed as many domains as you'd like. They also provide 4 DNS servers to add to your DNS listings:
To have your domain resolve correctly, please use ns1.everydns.net, ns2.everydns.net, ds3.everydns.net, and ns4.everydns.net for your nameservers. Note, NS1 is in San Diego, CA. while NS2 is in San Jose and NS3 is in the Netherlands and NS4 is in Washington DC.