Apple - What is SIMBL Agent and why is it running on my machine?

SIMBL is not a part of Apple's Mac OS X package, but is an extension that can be used by other developers in their own work to extend applications where they may not have the original source code (like Safari). SIMBL-related files can be found either in /Library/Application Support/SIMBL/Plugins or ~/Library/Application Support/SIMBL/Plugins.