@CBA
This is a 3rd Party Mod, providing Event Handler compatibility amongst other things, now shared among many add-ons and mods to ArmA2.
Also see CBA_OA.
Please see "Related" for cards tagged with CBA
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