The smallest and simplest client is DeeEnEss but that is good for DHCP connections only, it does not handle PPPoE logoff in any way.

For a fully featured client capable of updating many dynamic DNS services, and the DSL reports monitor as well, try DynSite. If you don't like either of these, try Dynamic DNS 4.0 or Dynamic DNS Client .NET Edition - Lite.

There is a page full of others at: dyndns.com

If you use any that have not been certified and they work, please let mjf know.