The art of making computers usable.
The thing that separates Mac OS from Windows and other operating systems. Not the same as interface design, although interface design is a part of HCI.
*Bruce Tognazzini (http://www.asktog.com), who usually has quite a lot to say about the state of interface design at Apple. Both Bruce and Donald have been Apple employees.
*Eric Raymond (OpenSource evangelist) has some things to say as to the state of interface design in the open source world: http://www.catb.org/~esr/writings/cups-horror.html
*Too lazy (and bandwidth-deficient) to find the link at the moment, but John Gruber should definitely be mentioned here, especially given his comments on Eric Raymond’s CUPS horrors. http://www.daringfireball.net/
*John Sirracusa on the Finder in Mac OS X (as opposed to the Finder in Mac OS): http://arstechnica.com/paedia/f/finder/finder-1.html
*There’s some (somewhat dated) articles at [http://www.mackido.com/Interface/index.html]