The jQuery Form Plugin allows you to easily and unobtrusively upgrade HTML forms to use AJAX. The main methods, ajaxForm and ajaxSubmit, gather information from the form element to determine how to manage the submit process. Both of these methods support numerous options which allows you to have full control over how the data is submitted.
jQuery Validation Plugin makes simple clientside form validation trivial, while offering lots of option for customization.
The plugin comes bundled with a useful set of validation methods, including URL and email validation, while providing an API to write your own methods. All bundled methods come with default error messages in english and translations into 36 locales.
jTwitter jQuery Twitter API Plugin provides you with an easy Twitter API to access Twitter user information by twitter username.