Arachnophilia is a simple IDE that works with a variety of programming languages. It's got a simple, but highly configurable, interface.
Arachnophilia is the primary IDE that I use in my classes. After starting my students at the command line, and after introducing them to other IDEs, about 80% of my students use Arachnophilia by choice, and are very productive with it. The other 20% usually choose to use the command line as their primary development environment, though they will usually use Arachnophilia as an adjunct, for example as a code beautifier.
I teach in a multi-platform computer lab. While this is a challenge, I mostly consider it an excellent opportunity for making a point to the students. The real world is a multi-platform environment. Since Arachnophilia is written in Java, it runs on all the lab's systems and behaves the same way on all of them. Arachnophilia includes a special install for Windows that makes getting it going on that OS easy. It even includes a nice icon. (To get an icon on OS X and other Unix systems, I get the favicon.ico file from Arachnophilia's web site and make that the program's icon.)
While I don't teach that Arachnophilia's interface is configurable, about one third of the class usually discovers this on their own and figures it out. They rapidly share this with the rest of the class, configuring their own build settings, customizing toolbars, and so on.
Arachnophilia behaves exactly the same on all platforms. This is a plus when you're calling out keystrokes in class, but it can be a problem for Mac users who are used to keyboard shortcuts that employ the Apple/Command key rather than the Control key. Moving back and forth between Arachnophilia and other apps on the Mac in the same session can be a bit disorienting as a result.
The interface the program provides is clean and easily accessible. I usually spend only about 10-15 minutes of class time getting students started with it, and another 10 minutes or so going into more advanced features later. Arachnophilia doesn't require any configuration to use before class (other than installation), unlike more advanced IDEs like Eclipse or Netbeans. Unlike Amaya, which is a nice IDE focused on HTML/CSS, we can keep using it when we move on to Java programming.
While I also like BlueJ a lot for class use, it is Java-specific, so I don't have an opportunity to introduce it during the HTML/CSS unit that I use as an introduction to programming concepts.
For my class, the ease of use and flexibility of Arachnophilia has made it the winner among a strong contingent of IDEs available.