jQuery and Mootools
Recently, I've taken the time to look into both the jQuery and MooTools Javascript libraries. The similarities and differences between the two have been expanded upon before in some depth, but I think that a more brief summary is in order.
jQuery does a great job of making plain HTML richer, especially if you make use of the accompanying jQuery UI library. If your client-side concerns start and end with providing a better user experience for HTML forms,then jQuery is your go-to guy.
On the other hand, you may plan on doing more intensive work, such as a client side application or browser based game for example. MooTools is great for this, especially if you are used to a different Object Oriented language such as Java or C#. The capability to design classes and methods in a familiar manner is an inestimably huge boon when attempting to build a sophisticated, maintainable program. (I never truly felt 'at home' with prototype based programming).
So there it is; jQuery for forms and MooTools for applications.
jQuery does a great job of making plain HTML richer, especially if you make use of the accompanying jQuery UI library. If your client-side concerns start and end with providing a better user experience for HTML forms,then jQuery is your go-to guy.