Interesting and good to know, thanks!
I always like to know about providers providing Secondary DNS service for “free” since I am such a proponent of using it.
So far my short list is:
Puck Secondary DNS (Free)
Hurricane Electric (Free)
1984 FreeDNS (Free - No Services Required)
Gandi NS6 (Free for domains registered via Gandi)
Hetzner also offers it but I have not found it to be overly simple to enable.
and now
Linode DNS Manager (Free with at least one VPS)
Anyone have any more that I can add to this list?