I’ve been using ZoneEdit.com recently, and its extremely simple to use, and best of all its free.  You can get five ‘zones’ for free, and each zone counts as a single domain, and then you can set up unlimited subdomains for each of these.

Good use case?  You can drop a Ning social network on your own domain, and only pay the $7/month ning software fee, plus the domain registration fee (maybe $1/month if that) .

This is much cheaper and easier to handle then buying a hosting account.  Most people don’t realize that DNS tools and hosting are separate.