1. QuickSelect
QuickSelect adds auto suggest type drop down functionality to textbox. It is very easy to integrate into your application and can work with a server side script using JSON. It can also convert a select box into auto suggesting dropdown textbox.
2. SexyCombo
This plugin turns existing select boxes into combination of text input and select list so that user can start typing into combobox to select an option.
3. prettyCheckboxes
This plugin makes sure that checkboxes and radio button look the same in different browsers.
4. jQuery Date Range Selector
If you need to accept range of dates from user, then this plugin is indispensable as it provides user with an intuitive way to select dates and date ranges.
5. jQuery Date Picker
If you just need to input a single date and not range of dates, then use this plugin as it makes inputting dates much easier for the user.
6. Modal Preview
Modal Preview lets users preview what they have entered into the textarea in a modal window. You can adjust the animation, opacity and set whether html is allowed in textarea or not.
7. Javascript Image Combo Box
With this plugin, you can create some great looking select boxes, as it allows you to add images next to each option of select box or you can even use only images as the select box options.
8. prettyComments
This plugin makes height of textareas dynamic based on length of the text entered into textarea.
9. Mark It Up
With MarkItUp, you can convert textareas into powerful HTML, BBCode or Wiki editor. With just one line of code, your textarea will become a full fledged editor.
10. jQuery Forms Plugin
Using this plugin, you can ajaxify any HTML form. It provides you with full control over how form data is submitted and how to handle response from the server.
11. Validation
The jQuery Validation plugin lets you easily define validation rules for form elements and it makes sure user only enters the correct input data. It can easily integrate with jQuery Forms plugin if you wish to use ajaxified forms.
12. Chained combobox
With this plugin, you can create stunning ajaxified chained select boxes using ajax call back and JSON response from the server.
If you would like to add some other useful jQuery plugin to this list, feel free to share them in comments below.