You can download and install the public beta version of the upcoming mountain lion messages app right now!
you can get the app from here
When you install Messages, it replaces iChat. But iChat services will continue to work. And Messages brings iMessage to the Mac â just like on iPad, iPhone, and iPod touch running iOS 5. Here are the features you can expect with Messages:
Send unlimited iMessages to any Mac, iPad, iPhone, or iPod touch.*
Start an iMessage conversation on your Mac and continue it on your iPad, iPhone, or iPod touch.
Send photos, videos, attachments, contacts, locations, and more.
Launch a FaceTime video call and bring the conversation face-to-face.
Messages supports iMessage, AIM, Yahoo!, Google Talk, and Jabber accounts.