Free DNS Online - My Experiences
Zone Edit : A great free DNS online service, 100% private and 100% free. Accounts used to be able to serve up to 5 domains, however recently this has been restricted to 2. The User Interface / Administration feels a bit clunky and old school web - but pretty straight forward and easy to use.
FreeDNS : Another good free DNS online service, however you entries can be reviewed by other users. Other users can configure subdomains off your domain, however you can pipe this through an authorisation process (where you the administrator are emailed and confirm any config changes). Payment is required if you want to make your listing private.
DNSExit : An uncapped free DNS online service. You can have as many accounts as you require... you can even set up dynamic DNS which is nice. Very slow though - and I have some questions regarding reliability. Recently the service was brought down by a DDOS attack for a couple of days.... have a backup - secondary dns on an alternate name server if possible.
DynDns : Free Dynamic DNS online service. Been around for years - can't recommend it enough for publicly serving from your dev box. Great for limited serving (i.e. demoing sites during development). Have a range of other paid services - however, a bit of googling for these will uncover alternative free services.