How to update element on view during onclick with RJS?

I am trying to set up a drop down menu. When the item on the drop down
list has been changed, I would like to display a textbox. Is there
anyway to execute RJS during in onclick or onchange event of the
select_tag?

Wai,

Check out the observe_field prototype helper:
http://caboo.se/doc/classes/ActionView/Helpers/PrototypeHelper.html#M005064

That should be the ticket.

Hope this helps,