noip.com is one of the few dynamic DNS free services that are reliable to use. If you have, like in my situation, a RaspberryPi connected to your home DSL and you want it to be always reachable without knowing the current IP address (the IP could change if you have a normal DSL service at home), you need a dynamic DNS service.
To update the noip.com one you just need ddclient a tool that is available in Raspbian/Debian repository. You can install it with this command:
sudo apt-get install ddclient
then you just need to edit /etc/ddclient.conf
protocol=dyndns2 use=web, web=checkip.dyndns.com/, web-skip='IP Address' server=dynupdate.no-ip.com login=yourusername password=yourpassword yourhostname.no-ip.org
and restart the client:
sudo /etc/init.d/ddclient restart
That's all! Please remember that noip.com free accounts have a limitation: they need to be confirmed every 30 days (you will receive an email and you need to click on the link contained to update your DNS).