Apple will bring FaceTime Audio to OS X in Mavericks 10.9.2
Following the public release of OS X 10.9.1 Mavericks, Apple has begun testing OS X 10.9.2 with developers to begin ironing out bugs. Other than the normal bug fixes and small features within the new version, it includes FaceTime Audio which will bring Apple's communication full-circle.
FaceTime Audio was introduced with iOS 7 in June, leaving out FaceTime's video functionality, allowing users to make calls to others without a phone number and using an Apple ID instead. According to 9to5mac, who has played with the OS X 10.9.2 beta, the FaceTime Audio feature is "deeply integrated" into the Messages and FaceTime OS X apps.
Essentially, Apple has a competitor to Skype on its hands, allowing users to communicate over text through the Messages app and make audio and video calls through FaceTime. Don't think Apple will be putting Skype out of business, however, as Apple's features are locked to iOS and OS X, where Skype reaches cross-platform.
Also included in the OS X 10.9.2 beta were minor tweaks to Mail. It's not clear when Apple might release OS X 10.9.2 to the public, but given the beta is in early stages, it could be a while.