Is this something loaded by Apple or did it come with some third party app I must have downloaded.
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
It is usually used by SafariFlvSniffer.bundle which comes with a video downloader app.