Currently: MAJOR.MINOR.PATCHLVL.BUILD
x.x.0 is (Pre) Release Candidate
x.x.1+ is Release Candidates, where the first one stable enough, is "The Stable Release"
Ongoing development 
ACE 1.13 RC5 [2011-12-09. Hotfix: 2012-04-27]
Stable
ACE 1.13 for OA [2011-12-23]
ACE 1.3 for A2 [2010-07-05] (Deprecated)
Dev blogs
Wheeled by Wagn v. 1.8.1