Here are few good (and some great) jQuery plug-ins that can boost your next proejct.

Cookie

Source:  http://plugins.jquery.com/project/cookie

Set and get cookies with jQuery.

Slideshows

There are lots of them, but this one is good in terms of the set of features.

http://jquery.malsup.com/cycle/ Slideshow plugin that supports many different types of transition effects.

Another option (simpler) is jCarousel:

http://sorgalla.com/jcarousel/ Control a list of items in horizontal or vertical order.

Date Range Picker

http://www.filamentgroup.com/lab/date_range_picker_using_jquery_ui_16_and_jquery_ui_css_framework/

Built on jQuery UI. You might want to use it only if the ‘default’ jQuery UI widget is not enough for you.

Flip

Simple & cool animation: http://lab.smashup.it/flip/

Flip your elements in four directions.

Flot: Charting

Source: http://code.google.com/p/flot/

A pure Javascript plotting library for jQuery. It produces graphical plots of arbitrary datasets on-the-fly client-side.

Form

Source: http://jquery.malsup.com/form/

Allows you to easily and unobtrusively upgrade HTML forms to use AJAX.

Hotkeys: Keyboard Bindings

http://code.google.com/p/js-hotkeys/ if you want your web app to be ‘reachable’ using keyboard – this is your plugin: Add and remove handlers for keyboard events anywhere in your code supporting almost any key combination.

Masked Input

Selection: http://digitalbush.com/projects/masked-input-plugin/

Allows a user to more easily enter fixed width input where you would like them to enter the data in a certain format (dates,phone numbers, etc).

Jcrop

Source: http://deepliquid.com/content/Jcrop.html

A quick and easy way to add image cropping functionality to your web application

jGrowl: Messaging System

Source: http://stanlemon.net/projects/jgrowl.html

Unobtrusive messages within the browser, similar to the way that OS X’s Growl Framework works

jqGrid

Source: http://www.trirand.com/jqgridwiki/doku.php

Represent and manipulate tabular data on the web.

JSON: Utility

Source: http://code.google.com/p/jquery-json/

Serializes a javascript object, number, string, or array into JSON (and a few other utilities).

JS Tree

Source: http://www.jstree.com/

jsTree is a javascript based, cross browser tree component.

UI Layout

Source: http://layout.jquery-dev.net/

Create any UI look you want – from simple headers or sidebars, to a complex application with toolbars, menus, help-panels, status bars, sub-forms, etc.

MarkItUp

Source: http://markitup.jaysalvat.com/home/

Allows you to turn any textarea into a markup editor.

Meta Data

Source: http://plugins.jquery.com/project/metadata

Extract metadata from classes, random attributes, child elements and HTML5 data-* attributes.

Popeye: Inline Gallery

Source: http://dev.herr-schuessler.de/jquery/popeye/

An advanced image gallery used to save space when displaying a collection of images and offer your users a nice and elegant way to show a big version of your images without leaving the page flow.

Quicksand: Animated Filtering

Source: http://razorjack.net/quicksand/

Reorder and filter items with a nice shuffling animation.

scrollTo

Source: http://flesler.blogspot.com/2007/10/jqueryscrollto.html

A small, customizable plugin for scrolling elements, or the window itself.

Spell Check

Source: http://github.com/brandonaaron/jquery-spellcheck

Adds spellcheck support to inputs. It uses Google’s spell checking API and requires a server to handle the communication with the API. An example php implementation is provided.

Star Rating

Source: http://www.fyneworks.com/jquery/star-rating/

Creates a non-obstrusive star rating control based on a set of radio input boxes.

Table Sorter

Source: http://tablesorter.com/docs/

Turn standard HTML table with THEAD and TBODY tags into a sortable table without page refreshes.

Uniform

Source: http://pixelmatrixdesign.com/uniform/

Uniform masks your standard form controls with custom themed controls.

Validate

Source: http://bassistance.de/jquery-plugins/jquery-plugin-validation/

Add client-side validation to your forms.