I get that this question is going to be one of those questions where everybody defends the app they like the most, but I am asking for the reasons why rather than just a one product name answer, so here goes with the question:
i.e. Qooxdoo, ExtJS, Echo3, etc?
Now I know everyone is probably going to jump in and say Jquery, but personally I see Jquery and the likes as more of a "make plain website nice by adding animations etc" rather than for web applications development.
Your thoughts please.... ;)