• ConTEXT: ConTEXT is a flexible and extensible editor that you can use with many scripting and programming languages. With this editor, you can have an unlimited number of files open and an unlimited editing file size. ConTEXT sports many features, including a custom-defined syntax highlighter, code templates, a macro recorder, bookmarks, search-and-replace capabilities, and a file explorer with a favorites list.
• Notepad++: Johan P.'s all-time favorite utility is a simple but useful editor named Notepad++. Notepad++ supports a wide variety of scripting and programming languages, including batch (.bat), VBScript, KiXtart, Perl, XML, HTML, Active Server Pages (ASP), and Visual Basic (VB). It offers features such as syntax highlighting and auto-completion.
• PSPad: The PSPad editor is on the recommendation list of John S. "I like it for Perl and use it daily." Besides Perl, PSPad supports many other scripting and programming languages. The features include syntax highlighting, code templates, a macro recorder, a full hex editor, a spell-checker, and the ability to handle plain text.
• SciTE: Adrian R. recommends the SciTE, an editor that's part of AutoIT v3. You can also download SciTE separately. Besides offering syntax highlighting, SciTE offers auto-completion and auto-indentation. Adrian notes that this editor will work with other scripting languages besides the AutoIt v3 language.
• Vim: If you write scripts in a cross-platform environment, you might want to check out the Vim editor. Michael V., who does scripting for both Linux and Windows systems, recommends Vim because, "Vim has the best of both worlds, with a command-line interface (just like UNIX) and a GUI for those who like menus and clicking. With a little effort, the editor can be tweaked to your personal likings with line numbers, fonts, and more for a seamless cross-platform workspace."

I write lots of vbscript using OnScript, from XLnow http://www.onscript.com/en/home.htm.  Typically, my scripts are for automation and developing application extensions.  Previous to encountering OnScript, I tried out a variety of scripting editors, and was always dissapointed with their abilities to expose the object model of the target application.  Most handled the Microsoft Office APIs OK, but not much else.  Most of my work involves Metis (aka Troux Architect), an object-oriented modeling tool, which has a COM API.  This is where OnScript particularly brings value for me, because it exposes the API as I'm editing scripts, does autocompletion, syntax highlighting, and all of the required editing features.  Additional features are tabs featuring library explorers, helps, script outline, command window, etc.  Its not free (about $100) but very much worth it.    

- Brent Nelson