Notepad++ is a popular general-purpose programmer's text editor for Windows that is great for plotscripting too! It is Free Software (GPL licensed), available in dozens of languages, and regularly updated.
Notepad++ has numerous useful features, like a tabbed interface (editing several files at the same time), and suggestions. However it isn't designed for whitespace-agnostic languages like HamsterSpeak, so you will get more use out of it if you write "walkHero" or "walkhero" instead of "walk hero".
A language definition (UDL) .xml file for HamsterSpeak can be downloaded at: File:Notepadpp-hamsterspeak.zip. This enables syntax highlighting and code folding. (Installation instructions are on that page).