Racket
Modern Scheme dialect. Matthew Flatt + PLT team, 1994 (then "PLT Scheme", renamed 2010). Famous for its macro system, "language-oriented programming", and Northeastern University CS teaching.
How to Design Programs (HtDP) is the intro book driving CS pedagogy around Racket. Built-in IDE (DrRacket). Typed Racket is the gradually-typed version. #lang racket + #lang typed/racket + custom #lang foo — every file can be its own language. Competitors: Clojure (mainstream Lisp), Common Lisp (industrial), Scheme (minimalism). Racket-on-Chez (2020) gave big performance gains by swapping runtime to Chez Scheme.