I've read the Newspeak on Squeak tutorial, but I could not find information or examples on mixins.
It must be quite trivial, as any class is also a mixin, but could you give me an example? (or a link)
The forum seems to be down at the moment, so I couldn't look there. http://newspeaklanguage.org/forums/
There are no mixin examples yet.
Newspeak is very experimental. I use the Agda programming language that is experimental too, but Newspeak is even more experimental. There is very little documentation and one thing that bothers me is that there seems to be no support for 64bit architectures, so you have to install all the extra 32bit libraries in linux. Also there is no date when 64bit support will be available. I am further not sure if there is or will be multi threading support.
If you are looking for experimental and good programming languages, check out Agda, Coq, F* (from MS), Isabell/HOL, and Haskell.
Newspeak would have been a good language if it were already finished. I fear that it will get too much behind by the time the language is usable.