For the server on which this web site runs, I used the domain registrar's (GoDaddy in this case) DNS servers. However, this caused a few problems, so I looked for another solution.

1. I couldn't convince GoDaddy's system to accept my own DNS server as the primary server. Since the Plesk-controlled DNS server on this web and mail server gets most data automatically from Plesk (domainkey entries, to give just one example), I really wanted this to be the primary DNS server.
2. GoDaddy doesn't quite give me full control over the DNS entries. My own server is more flexible.
3. GoDaddy's web interface is a bit on the slow side and not quite so good as my own.

So I looked for secondary DNS servers, and indeed there is a small number of companies who offer them as a free service. They do this on the side as an advertisement, quite an effective one in my case, for their other services, like web and mail hosting and professional, large scale, paid DNS services.

I found a list of free secondary DNS services and tried four of them with the following results.

1. vcsweb.com.au – with two servers that so far work pretty well for us.
2. twisted4life – this apparently British company offers the nicest web user interface of the bunch and a good, though very slow DNS configuration checker (see below). On the other hand their single server is slow and unreliable, and they take only 10 domains for free. Still we're using it now.
3. everydns.net – four servers, one of them ostensibly in Holland, but I only saw two or three working. Their service definitely works, and you can get DNS for up to 20 domains for free, but I ended up not using them, partly also because of their poor and minimalistic web user interface.
4. afraid.org – I had used them before for primary DNS services, but their web user interface is so idiotic that I stopped trying before I even got the first domain set up.

DNS check

vcsweb links to this DNS configuration checker.

twisted4life has this nice looking, but very slow DNS configuration checker, which can replace the earlier DNS Stuff service, which is much better, but no longer free.▼

All of the free services are slow. Some promise to update their servers hourly, but I found that this is usually a lie. These promises may have been kept when the servers were not yet heavily used, but all of the free services are apparently quite loaded. Still they all seem to be usable for secondary DNS server purposes, when you consider that they are only very rarely actually used.