"jQuery is another excellent JavaScript framework that followed prototype. Its motto is — WRITE LESS, DO MORE, meaning write less code and accomplish more tasks. It's a lightweight js library (only 21k when compressed), which surpasses other js libraries in this aspect. It's compatible with CSS3 and supports various browsers (IE 6.0+, FF 1.5+, Safari 2.0+, Opera 9.0+). jQuery is a fast and concise JavaScript library that makes it easier for users to handle HTML documents, events, implement animation effects, and provide AJAX interactions on websites. One significant advantage of jQuery is its comprehensive documentation and detailed explanations of various applications, along with many mature plugins available for selection. jQuery can help keep your HTML pages clean by separating code from content, meaning you don't need to insert a bunch of JS commands into the HTML anymore; you just need to define IDs."
Here provides over 240 jQuery plugins. There are really too many! As follows:
Form Validation
jQuery Validation.Auto Help.Simple jQuery form validation.jQuery XAV - form validations.jQuery AlphaNumeric.Masked Input.TypeWatch Plugin.Text limiter for form fields.Ajax Username Check with jQuery.
File Upload
Ajax File Upload.jQUploader.Multiple File Upload plugin.jQuery File Style.Styling an input type file.Progress Bar Plugin.
Forms - Select Box Stuff
jQuery Combobox.jQuery controlled dependent (or Cascading) Select List.Multiple Selects.Select box manipulation.Select Combo Plugin.jQuery - LinkedSelectAuto-populate multiple select boxes.Choose Plugin (Select Replacement).
Form Basics, Input Fields, Checkboxes etc.
jQuery Form Plugin.jQuery-Form.jLook Nice Forms.jNice.Ping Plugin.Toggle Form Text.ToggleVal.jQuery Field Plugin.jQuery Form’n Field plugin.jQuery Checkbox manipulation.jTagging.jQuery labelcheck.Overlabel.3 state radio buttons.ShiftCheckbox jQuery Plugin.Watermark Input.jQuery Checkbox (checkboxes with images).jQuery SpinButton Control.jQuery Ajax Form Builder.jQuery Focus Fields.jQuery Time Entry.
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Source: http://www.kollermedia.at/archive/2007/11/21/the-ultimate-jquery-plugin-list/