451138 Javascript select form

I need a javascript that manipulates form objects. I need a select object with categories that populate a subcategory select object. It should allow easy updates, and at least 3 levels deep. I will eventually populate all categories, subcategories and sub-sub categories from a database(After you are complete). For example, it needs to work to like this: (Feel free to use your own naming convention): When option A is selected from a select object named SelectLevel1, a second select object(SelectLevel2) should appear with options A1, A2, A3, etc. When option A1 is selected, a third object(named SelectLevel3) will appear with options A11, A12, A13, etc. Feel free to use jquery or whatever technique you'd like. It must be compatible with IE6, IE7, IE8, IE9, Safari, Firefox 3.x, etc.