iMessage (the free iPhone-to-iPhone messaging service that is integrated into the SMS app on your iPhone running iOS5) is now also accessible from your Mac. Apple has released a Mac-version called “Messages” that is ‘available for beta-testing’ (i.e. ‘free to use’, but it might still have some bugs and shortcomings) :
http://www.apple.com/macosx/mountain-lion/messages-beta/
Using it is quite straight-forward, as you just need your AppleID to login and the interface resembles both the iPhone’s SMS-app and iChat’s text-chatting.
Messages will be part of Apple’s next MacOS (called OSX 10.8 “Mountain Lion”) which will be released this summer. The beta-version will run on MacOSX 10.7 “Lion” only, and will stop working after the introduction of OSX 10.8 “Mountain Lion”. (A procedure we’ve seen before with other beta-applications like FaceTime for Mac.
So with the release of MacOSX 10.7 “Lion”, iChat was abandoned, but FaceTime only replaced iChat video-chat. With the release of OSX 10.8 “Mountian Lion”, Messages will revive what was formerly know as iChat text-chat…
MacManus.nl 
Yes, you’ve read it correctly : as of the next version of the MacOS, it will no longer be called “Mac OS X”, but just “OS X”…
Just like “iPhoneOS” was replaced by “iOS” at the introduction of the first iPad…
…if and how this name-change will affect “Apple’s commitment to the Mac” (one of the last emphasized statements of Steve Jobs) is unclear…
Apple’s ’emphasized statements’ often are like ‘magician’s decoys’…
…remember “Apple’s commitment to the PowerPC” (while Apple started porting MacOSX to Intel-processors during the early development of OSX in the late 90s), “We are not working on a mobile phone” (while Apple was already developing the iPhone), “We are not working on a tablet, there is no market for those” (less than a year before the introduction of the first iPad), “13-inch is the smallest usable screen for a notebook” (and not too long afterwards the 11-inch MacBook Air was introduced), etc. etc….