Here is a list of few popular Multi Instant Messengers for Mobile Phones. I know there are many other but these are the popular ones. They support all the Popular Instant Messengers like Google Talk, Yahoo Messenger, Msn Messengers, AIM etc.

Numbuzz

Nimbuzz is among my favorites. Not only it supports all the poplar messengers, it also lets you make VoIP calls. You need to buy credits for making calls though.

I love the Interface of Nimbuzz more than any other application available in the market. Plus it is available for Android,Symbian and also Java phones. 

Download Nimbuzz from Here

eBuddy

eBuddy is the 2nd best according to me. Runs quite smoothly in my phone. It is also available for iPhone, Android Powered mobile phones, Symbian and Java Enabled phones. Supports MSN, Yahoo!, AIM, Gtalk, ICQ and Facebook. eBuddy also offers web version where  you can use all these messengers in a web browser in PC.

Download eBuddy from here

Palringo

I just downloaded it on my Symbian Mobile phones and it looks nice to me. It is available for Symbian Phones, Java Phones, iPhone, Android Phones and even for Blackberry and Windows Mobile Phones.
Apart fro instant Messaging, you can do picture and voice messages via palringo, but you need to upgrade to premium account.

Download Palringo from here

Fring

This is another Application I have been using from a long time. The application is quite nice and does not confuse you by giving too many options, no display icons of my friend list. The message tone annoys my to the core, You can however change it to your favorite. Fring is available for Android Phones, Symbian Phones and iPhone.

Download Fring from here 

Trillian

Trillian is available for iPhone, Android and Blackberry phones. It is just like any other multi messenger application for mobile phones, lets you chat with your friends on the go. But if you are an iPhone users, you will have to pay $4.99 to download this from Apple Store. Its $4.99 for Blackberry as well.

Download Trillian from here

Agile Mobile

Agile mobile messenger is available for iPhone, Java Phones (Nokia and Sony Ericsson Phones) and Blackberry phones. Will cost $9.99 for iPhone and Comes liscence for lifetime for $44.95.

Download Agile Mobile from here

Beejive

Beejive is another popular Multi IM application for iPhone, iPad and Blackberry, it will however cost you $9.9 for both iPhone and Blackberry.

The main feature of this application is that you can send SMS over data plans which means you will not be charged for SMS from your carrier.

Download beejive from here