One of the most powerful features of the framework is that it's been developed with a highly extensible plugin model.
We recently decided to add some interactivity to our partner site Bluesulphur to replace some of the flash content we currently have. As part of that, I looked at some of the plugins currently available.
These are some of the nicest plugins I found.
Facebook search engine - A Facebook style autosuggest plugin using jQuery. It's particularly pretty and although it currently uses a php source for data, this isn't a big limitation, these can be swapped out to use .NET web services as a provider quite easily.
coda bubble is a mac like popup that could be put to use in many situations.
jQuery Tablesorter is a cute little plugin that makes any table sortable
Fancybox is a popup plugin that you can use for popup image galleries, the popups have nice transition effects as well as inbuilt navigation and grouping functionality.
I thought for the last example, I'd show an html editor and I really like markitup, it's both simple and sexy. It looks like it's an easy integrator too, which is always a plus.
So, to sum up, there are a lot jQuery plugins out there, more than 2500 just on the jQuery plugin site alone. New and awesome plugins are being released every day, check them out!
Oh, and Compsoft are a great choice for integrating jQuery interactivity and plugins into your site too! *wink*