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Most of the applications and enhancements that you have looked at so far have been created from scratch using the default functionality of jQuery. One of the most valuable and powerful features of jQuery is the plug-in structure, and many incredibly useful and beautiful plug-ins are being added to the list every day.
Most of the plug-ins have not been built with SharePoint specifically in mind, but because of their configurable nature, getting a plug-in to work in a SharePoint environment is just like adding it to any other HTML page. The benefit is that you have the SharePoint API on hand, too, to help work with your data.
The plug-ins available range tremendously in the value they offer. Arguably, the most utilized is jQuery UI, which adds a host of UI enhancements to jQuery. Another example is SPServices, which provides a fantastic jQuery wrapper around the SharePoint web services. There are plenty of other plug-ins that you may find useful.