The Smart InputManager Bundle Loader (SIMBL), developed by Mike Solomon, is a hack, and not only that, it’s a hack that allows for more hacks. In this meaning, it’s a good hack. SIMBL allows developers to write plugins for cocoa applications that would normally not allow them. Once SIMBL is installed, each time a cocoa app is launched, it scans the SIMBL plugin library (/Library/Application Support/SIMBL/Plugins) to see if there is a plugin. If one is found, the app loads the plugin and continues launching.
The reason it’s a hack is that it allows modification to running programs that the developer does not have access to the source code to… they have to guess. Luckily, there is so much documentation about cocoa, that the developers can make a very educated guess. The reason its a good hack is because it modifies the existing InputManger to allow the plugins to load only for thier intended application, instead of all cocoa apps. This means developers can target a particular Apple app, like, say, Safari.