a readonly field with a button near it, on button click a window pops up, where user can search and select a value

Lookup with pagination (more results button)

bound to ajaxdropdown using parentId

unlike the AjaxDropdown the Multilookup will keep its value when the value of its parent is changed
can be bound to anything else HiddenField, TextBox, Multilookup etc.

Bound to 2 dropdowns (using Data)

send predefined parameters, and fullscreen