Nimbuzz

Nimbuzz is a multi-chat protocol client that works on most devices available today. It is compatible with all the popular protocols such as Yahoo!, MSN, Facebook and so on.

It even supports protocols that are hardly used such as Hyves. And if this wasn't enough for you, the developers are constantly adding support for other protocols with each new version.
Nimbuzz is useful for people who need to use a lot of chat protocols. Instead of opening multiple applications, just open Nimbuzz, create an account, add the chat protocols you want and you are done. And what's more, Nimbuzz also supports voice calls from Yahoo, MSN, etc.
If you are comfortable with Nimbuzz and do not want to stop using it then you are in luck because Nimbuzz also supports other platforms. You can now chat from your mobile phone, smartphone, etc. You do not even have to create another account while using Nimbuzz on other devices since you can use the same account anywhere.
Add the fact that Nimbuzz is very lightweight and user-friendly and you get a chat client that you will never want to live without again.