For years I have used secondary.com as my provider of secondary DNS services.  These guys were then acquired by a company called Neustar who specialize in DNS for the enterprise.  This was no problem and things ran great until I heard that Neustar was shutting down their free DNS services.  This was too bad given my positive experience over the years.  They offered to continue my service albeit with a $50/month fee and a very complex metering structure.  Thanks, but no thanks.

I completed a thorough analysis and decided to migrate DNS to free service called everydns.com.  The new service is currently up and running and the migration should be transparent.  So far so good.  I recommend these folks to anyone needing a free DNS provider although they do request a donation.