Doh, someone beat me to it. For every one of your requirements, especially the multiplatform, and the fact that they are small programs, Notepad++ is the way to go! I don't really program much, but when I do I use TextPad because that is what we have at work. It's clip library is a great feature as well that really makes it worth using if you are only on Windows.
When I do similar things at home, I found Notepad++ almost as easy to use and it is free (and portable). Many of it's features are actually slightly nicer in my opinion to TextPad, but I haven't found a clip-library type of feature that is comparable. I miss this greatly when I use Notepad++ as it speeds development significantly. I suppose I could create something similar on a different tab, but it still isn't the same. If the clip-library isn't a big deal to you, then I would say TextPad isn't the tool you are looking for.
As for Netbeans, I used it in the past, so I though I am talking about it, it may have expanded significantly since I used it last. That said, my experience with Netbeans is it is great if you are making Java specific GUI's. Otherwise it is just another Java platform that works well, but not exceptional. It definitely does not (well did not) do all required languages easily or treat them equally well. Moreover, it is (was) rather bloated as IDE's go. In general, Notepad++ (and other similar apps) are over glorified Text Editors that have some of the most common IDE features to extend their capabilities. Netbeans, Eclipse, et. al. are more full featured and purpose specific; but do not really add much for the small programs and/or languages you talk about. In fact, even in school, I took a Java course where the instructor specifically suggested TextPad or Notepad++ because we would not use the features of Netbeans until the last week of school and they were far more difficult to setup and use. The only feature we used during that last week of school was the GUI creation, because you can put it together visually, then code the functions of each section later. That was all we used Netbeans for. TextPad/Notepad++ was used for everything else in the curriculum.