QwertyWEB

              Only one msn , hotmail or live  account can be opened [for chatting] in one OS unlike Gmail and Yahoo , not if considered via opening in only one browser , but for chatting , only one account id can be signed in , and if you open another id through browser , the windows messenger gets logged out ! and signed in through the account used in browser !


For this ,
Messenger Plus live is required !
It's a free add on [but ads are shown at startup] integrated with Window Live Messenger enhanced with  hundreds of features and many  possibilities of Messenger and make your experience a lot more entertaining!

The best Main feature i love this Addon is that i can chat with many idz all at once , as i have 4 different idz , i can open all at once and easily chat ! , which was not possible without this addon .

Other great features are ,
 
1>Log Search
Messenger Plus! now lets you search straight from the Log Viewer, indexing all your conversations automatically.

2>Messenger Skinning

• Completely change the look of your Messenger windows with easy-to-install skins.
• Recent contact events appear in conversation windowsContact List Clean-Up now lets you view a list of the contacts that have you in their list, random sounds can now be added from the Sound Library window

3>Tabbed Chats

• Use a tabbed web browser? why not use the same thing for your conversations?
• All your chat windows can now be grouped into a better single one. 
• Different positions, different options, this feature is yours to customize. 

4>Custom Sounds

• The Custom Emotion Sounds feature allows users to send their own pre-recorded sounds during a conversation. 
• Sounds are played instantly on both ends and are transferred automatically and transparently when needed. 
• Millions of sounds are already available in every category imaginable, all you need is a couple of sound packs to start your private collection. 
• New emotion sounds can also easily be created from existing sound files (such as MP3s) or recorded live from a microphone or another audio source.

5>Messenger Lock

• Be it for security/privacy in shared computer environments or for avoiding your boss (which would be wrong... ;-), this feature lets you lock Messenger with a simple keyboard shortcut and hide it in the system tray with an inconspicuous icon. 
• Messenger can then be password protected and messages can automatically be sent to your contacts to inform them of a lock or unlock.

Note :
It works only for Windows !
And of course , Its free ! , but you should have latest version of msn messenger , otherwise you will need to update it , but if you are using pirated version of windows OS , you are bound to get screwed :P
For genuine windows users and for those who have made it genuine , here's the link
Download Messenger Plus Live