How to get OS X Server (for OS X Mavericks) for free (as Mac or iOS Developer)?
9 To Mac states this clearly:


  To download OS X Server, developers should access the iOS or Mac developer portal to retrieve an App Store redemption code.



If you have iOS development program go to:
https://developer.apple.com/osx/download/
Scroll down
Click Get download code 
Redeem the code
Download your free version of OS X Server 3.0.3


This text is next to the button:


  OS X Server 3.0.3 Includes Xcode Server to enable continuous
  integration. Works with Xcode 5 to configure bots that automatically
  build and test your apps. Requires OS X Mavericks.