There is a huge collection of plugins available that extend the base framework with new functionality. One of the most popular of these plugins is jQuery UI. It provides additional abstractions over low-level interaction and animation, advanced effects and high-level themeable widgets for building highly interactive web applications.
jQuery and jQuery UI are both well integrated into Seaside 3.0. This allows you to access all aspects of the library from Smalltalk by writing Smalltalk code only. The Smalltalk side of the integration is automatically built from the excellent jQuery documentation, so you can be sure that the integration is up-to-date and feature-complete.