Complex sentence: On Safari (and other Webkit applications), there is a SIMBL-managed plug-in called GreaseKit, for Safari 5 there is an extension called NinjaKit that uses the new API interface.
Split into simpler sentences: On Safari (and other Webkit applications), there is a SIMBL-managed plug-in called GreaseKit. Since the release of Safari 5 there is an extension called NinjaKit that uses the new API interface.