Also, DrRacket. While I generally prefer my emacs, "first learn emacs" is a pretty bitter pill to swallow for newcomers. Fortunately DrRacket solves that for both newcomers as well as experienced folk who just don't want to use emacs. Introducing folks to Racket seems to have a lower impedance threshold as a result.

DrRacket looks a little dated in its vanilla configuration, but the layout of the editor / REPL parts can be changed around a bit, the color schemes swapped out. There's even a paredit mode for DrRacket. Overall, if I were somehow coding more exclusively in Racket, I could easily see using and extending DrRacket rather than my current emacs.