Hurricane Electric's Tunnelbroker provides free 6in4 IPv6 tunnels. DNS-O-Matic provides a single Dyn-compatible interface to update all of your dynamic DNS resources. I use DNS-O-Matic because I want to update DynDNS, OpenDNS, and Tunnelbroker, all at the same time. I had a hard time finding the correct configuration for Tunnelbroker on DNS-O-Matic. There's conflicting information because Tunnelbroker also provides a non-Dyn-compatible interface that uses different data. Their Dyn-compatible endpoint is at https://ipv4.tunnelbroker.net/nic/update.
User ID: Your Tunnelbroker Username (Account Name), the one you use to log
in, not your long hashy User ID as suggested elsewhere.
Password: The Update Key from your tunnel's information page, or your Tunnelbroker Password (the one you use to log in), if you don't have an Update Key.
Host/Identifier: The DNS fully-qualified name of your tunnel, not the numeric Tunnel ID. This is of the form