Dynamic dropdown menus

[eluser]phester[/eluser]
This is not really a purely CI related question, but I was wondering if any of you have a quick easy solution!

I am trying to create 2 drop down menus where the options in the second menu is dependent on the value selected in the first menu. For example, let's say the first menu has the following options:

BMW
Mercedes
Lexus

The second option will have the following options depending on which value was selected:

If BMW was selected:
3 Series
5 Series
7 Series

If Lexus was selected:
IS
ES
GS
LS

And so on...

I am curious for a simple javascript solution using AJAX with jQuery or Prototype. I would like the options of the second menu be fed from a client side script using AJAX.

Any help is greatly appreciated!