The library I hear the most about is jQuery. However there are a whole lot more. Developers sometimes think that ExtJS does not get enough press.
If you are not integrating it into a commercial product, you can get ExtJS for free. You add some lines of code to your header section to bring in the ExtJS functionality. Then you use the extension library framework.
Salary Comparison Failure - Read a post that stated top bug bounty hunters make 3X the salary of average developers. Umm what? Who cares what those top people make? You got to compar...