Userscripts is described as 'An open-source userscript editor for Safari' and is an app in the Development category. There are four alternatives to Userscripts for a variety of platforms, including Firefox, Linux, Mac, Windows and Opera. The best alternative is Greasemonkey, which is both free and Open Source. Other great apps like Userscripts are Tampermonkey (Free Personal), Violentmonkey (Free, Open Source) and Launchlet (Freemium, Open Source).
Greasemonkey is a Firefox extension that allows you to customize the way webpages look and function. Hundreds of scripts are already available for free. And if youre the tinkerer sort, you can also write your own.