List with sub categories?

Hello,
In my registration form I would like to create a drop down list with 15 items, now once a user select an item from list another drop down list appears with another set of items that is unique to the selection they made from the first list(of 15 items). I hope this is clear, appreciate any help setting this up if possible.

Only thing that can currently do this is our CB Conditional plugin incubator project which is available (with 32 other add-ons) to Professional members.

More information about add-ons offered for professional members:
www.joomlapolis.com/cb-solutions/add-ons
www.joomlapolis.com/cb-solutions/incubator

and a review of CB Conditional plugin:

www.joomlapolis.com/cb-conditional-short-review