SIMBL (short for SIMple Bundle Loader, formerly Smart InputManager Bundle Loader, and pronounced like symbol or cymbal), is an application enhancement (InputManager bundle) loader for Mac OS X developed by Mike Solomon. It helps third-party developers modify and add functionality to applications developed with the Cocoa environment without access to the source code. SIMBL loads code via the InputManager system, which was developed to support foreign input methods. Plugins using SIMBL have advantages over normal InputManager modifications such as targeted code loading into specific applications. Designed for Solomon's PithHelmet, SIMBL is now used by other developers. The most popular use of SIMBL is to add functionality to the Apple Safari web browser which did not have an Apple-authorized plugin system until version 5 in 2010.
SIMBL was developed in response to the restrictive licensing and costs of Application Enhancer and the drawbacks of loading code blindly through the InputManager mechanism.