Mindy was used to bootstrap Gwydion Dylan, a Dylan compiler implementation. This was the main open source Dylan implementation for quite some time until Functional Objects (which obtained the code and rights from Harlequin when they went out of the language business) open sourced their implementation which is now 'Open Dylan'.

Functional Developer (What was Harelequin Dylan) was nice. The editor would syntax highlight the code based on the optimization level. In this way you could look at a method call and see if it was inlined or not and decide whether to seal functions or classes to improve performance.

Another interesting feature was the editor was written in Dylan (using a framework called Deuce). This was emacs-like and had features like virtual sections in a buffer that could each be mapped to a different file on disk. This allowed viewing in a single editor pane the implementations of all methods of a generic function even if they crossed multiple files. This could also be edited as if it was a single file.