JQuery has exploded into omnipresence over the past 2 years. Javascript libraries have been around for ages, but jQuery seems to have hit a lot of the right chords in the webdevelopment community.

Apart from it's low learning curve, it's probably the fact that jQuery tries to function more as a platform for invention and creativity than as a substitute for these, that has allowed it to gain such a wide user base in such a short period of time. The abundance of plugins and scripts written are both a testimony to its present popularity and a stimulus to its future development.
Although it's always more interesting to code ones own solutions, sometimes it's nice to find someone has already done all the work for you. Below is a list of jQuery plugins and applications that I've found particularly useful.
- jQuery form plugin - http://malsup.com/jquery/form/
A very complete code base for handling all you forms submits and corresponding responses with ajax and iframe implementations. I use it on almost evey project I embark on nowadays. - jQuery cycle - http://malsup.com/jquery/cycle/
A very complete and highly configurable library for animated cycling of images or other content with. - jQuery ScrollTo - http://demos.flesler.com/jquery/scrollTo/
Animated scrolling in every way imaginable. Very easy to implement and allows many different types of scroll target -specifications - jQuery UI Sortable - http://docs.jquery.com/UI/Sortable
Allows you to sort, reposition and even change parent element for jQuery objects, all via drag and drop. The handiest drag and drop implementation I've come across so far. - jQuery Flash Plugin - http://jquery.lukelutman.com/plugins/flash/
Does nothing special. Just makes including flash elements on your pages even easier and more manageable.