Horizontal navigation is now available on Google Sites. See the instructions here.
See Steegle's Google Sites - Horizontal Navigation with drop-down Menus information here.
Someone has created a new horizontal navigation gadget. His forum posting is here.
With this gadget you can have left, right or centered menus but since you can't put gadgets in the header you will have to insert the gadget in each page where you want it to appear.
To insert it into a page use Insert, More Gadgets, select Add gadget by URL and insert this URL:
Here is an example:
See also this menu by Mori: