I am probably at the edge of etiquette here since the question is somewhat subjective, and not stricly about programming, but usability - but in the end, what is more important than the usability of the solutions we create?
The idea is to "work backwards" from usability to technology:
I want to see an impressive Web-GUI (show URL if possible) and then learn what technology enabled it.
never seen such a usable site
They are using:
I think you should take a look at jquery there's a lot of sites (amazon, google, IBM) using this framework this works pretty fine and also has a lot of plugins that make your life easier. If you want an impressive UI I think you need to take a look to it.
Here is the link if you want to check some sites using Jquery http://docs.jquery.com/Sites_Using_jQuery
Usability is technology independent largely.
Usability is about human factors, user centred design, user performance. The best websites and technology are the one which allow users to perform at an optimum level, achieving their task objectives within a context they understand (i.e. language, culture, etc.)
A list of URL's for examples of goo dusability is nice. But please don't take their design and implement it in some other context and think you are accommodating for good usability.
Focus on the task, context and user -> then design for that.
Good luck!