ACT (AjaxControlToolkit) and JQuery offers gave UI flexibility for asp.net developers. ACT can be used for WebForms while JQuery can be used for ASP.NET MVC. Here are the equivalent controls in ACT and JQuery that are popular:

ACT TabContainer and JQuery Tabs

ACT CalendarExtender and JQuery DatePicker

ACT Accordion and JQuery Accordion

ACT ReorderList and JQuery Sortable

ACT ModalPopupExtender and JQuery Dialogs

The key difference between the two is that ACT is based on Microsoft AJAX client scripts whereas JQuery is purely javascript based and used by popular websites including Google.