In Mountain Lion (OS X 10.8.x) the app is now called Messages.
This adds in the iOS ability to iMessage (Apple's own Messaging service) to iPhones and other iOS devices (As well as Mountain Lion computers).
The ability to use AIM and Jabber is still in the app although Buddy lists are not on Show by default.
Add either an AIM or Jabber ID to the Messages Menu > Preferences > Accounts (Use the + button) depending on what you are using yourself.
As you know some Apple IDs are valid AIM names (This may mean adding it as the iMessages ID AND as an AIM account in the Add Pane ).
Only the accounts that can do Audio and Video can do Screen Sharing.
This excludes the Yahoo option and the iMessages account (this invokes the FaceTime app instead).
The internet speed required has to be above the 128kbps in both directions (If you can Video chat you can Screen Share).
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously