Scriptish is described as 'forked off Greasemonkey because enough developers felt its development had grown stagnant' and is an app in the Development category. There are four alternatives to Scriptish for a variety of platforms, including Firefox, Windows, Linux, Mac and Google Chrome. The best alternative is Greasemonkey, which is both free and Open Source. Other great apps like Scriptish are Tampermonkey (Free Personal), Violentmonkey (Free, Open Source) and Scriptify (Free, 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.