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....
jQuery can be used for even the most complex web applications, especially if you bring in the extras like jQuery UI and the myriad plugins that are out there. Its maturity, enormous user base, good documentation and simplicity are what would encourage me to use it.