Application Enhancer

Application Enhancer (APE) is a software program released by Unsanity for Apple's Mac OS X operating system. Application Enhancer provides a framework that allows third-party developers to write "haxies" for Mac OS X. It also provides a daemon to load haxies when certain applications are launched. These haxies, or plugins, are known as application enhancer modules, or APEs. Once the APE is loaded by the daemon, the module modifies the behavior of an existing application. Examples include allowing the classic Mac OS WindowShade behavior, or adding/removing Mac OS X's brushed aluminum theme to/from all applications.
Application Enhancer is freeware, but modules may be distributed as freeware, shareware, or commercial software. A license for the Application Enhancer SDK (required by developers to deploy haxies) is $100 for shareware products and $1000 for commercial products.
Leopard compatibility issues.
With the release of Mac OS X v10.5, users running an outdated copy of APE (2.0.1 or earlier) began to report problems with their upgraded Leopard systems. The problems occurred when using the "Upgrade" option rather than the "Erase and Install" or "Archive and Install" option. Symptoms included hour-long "blue screens" (known as the 'Blue Screen of Death') on restart and systems hung inoperable for hours at a time. Users gradually discovered the error and were able to fix it by updating their installation of APE. On 27 October 2007 Unsanity alerted mailing list subscribers of the issue and advised them to have the latest version of the Application Enhancer framework installed prior to installing or upgrading to Leopard.