Nokia. As you chat with Yahoo Messenger without having to install anything.

.
Not everyone knows (including myself), but on 5800 and other Nokia smartphones equipped with a series of im client, you can use the instant messaging client default to talk with our contacts in Yahoo Messenger.

Why this happens, simply enter the data of your yahoo account (user, password, etc.). Imps.msg.yahoo.com and enter as the address of the server. From now on, you can also chat with users of Yahoo Messenger;)