MeddleMonkey is the user script manager which supports Safari. This extension supports features such as easy script installation, automatic update checks, a simple overview of the scripts running on the tab, and has a built-in editor.
MeddleMonkey Safari extension greatly simplifies the process of managing custom scripts. The MeddleMonkey icon in the upper left corner shows the number of running scripts, and clicking on it opens a list of already running scripts and a list of scripts that can be run on this page.