Long time ago I got a DynDns.org address, as it was the only thing my Linksys router supported. They then stopped the free service for new users, but allowed existing users to remain free. To their credit they have kept their promise for a number of years, but this week I got an e-mail informing me I must log into their website every 30 days to keep the domain.
This is obviously just a polite way to get rid of free users, as I doubt anyone will want to log in that often. So I made the switch over to freedns.afraid.org (http://freedns.afraid.org) who I have been using as a free secondary DNS on my websites. At home my dynamic address is updated by my DD-WRT based router, and for my parents home, where the router is more basic, I use a free client that sits in the tray at logon (http://freedns.afraid.org/scripts/freedns.clients.php).