Are there Greasemonkey scripts for Safari? There's the wonderful Greasemonkey to run user-defined JavaScript on web pages.

Is there such a thing as Greasemonkey for Safari? Or would one instead write a Safari Extension to customize behavior on other web pages? Maybe there's an alternative that runs on AppleScript instead of JavaScript?
Another option to run GreaseMonkey scripts is NinjaKit (which is what I use). It's a Safari 5 extension, which means it's just a tad safer than GreaseKit.