postgres, python, inkscape, XFCE, various shells, GTK, Qt, even libreoffice (which has gone through astounding refactoring), vim, emacs, apache, nginx.
That's a good point. Maybe it is a matter of perspective: none of those are consumer programs, they are developer tools. (Except libreoffice, but that has had its problems.)
So you could argue that they are not all that user friendly.
> Contrary to popular belief, Unix is user friendly. It just happens to be very selective about who it decides to make friends with.
They are user friendly they just represent the usages of the type of user using them.
This is something I see said a lot, constraining an interface to your average completely new user who knows nothing makes it very difficult to have that interface adequately serve people with more experience, you load up the benefit on the front side and trash the backside in response - that said some powerful interfaces are just objectively bad but the tools utility overrides that.
user friendly - designed to aid the user in doing whatever task they are doing.
The software for making a photo collage for Grandma and the software for controlling replication across a hundred database nodes can be radically different and still be user friendly.
Lots of software tends to assume that the user is a new user but the problem then is that a new user rapidly becomes a not-new user and those things that helped initially now hamper productivity (cough Clippy cough).
I'm not saying everyone should have a Symbolics terminal and that requiring lisp knowledge should be mandatory so much as lots of people say open source isn't "user friendly" when what they mean is it's not particularly friendly to new/inexperience users coming from outside.
http://rippedwire.sourceforge.net/images/hbgtk-video.jpg for example, to a new user that is very unfriendly but if you use it frequently and or understand what you are trying to do that kind of no-nonsense interface is user friendly.
I don't think there is a rule.