Meebo

www.meebo.com is about instant messages. With AIM, Yahoo, Googletalk and MSN, there is no doubt about it, there are too many different systems.

But visit meebo.com and you can connect to them all over the web. So you can use them all at the same time without having to download all those different applications. Putting it to the test, I must admit I felt uncomfortable giving away so many passwords and usernames all in one go.
But, having said that, this is a great way to get in touch with all of your friends through one single portal, no matter what system you use. And let us not forget, it is free and it works. There are not many things you can say that about, but here is another one