Horizontal navigation is now available on Google Sites. See the instructions here. Please note that if Compatibility View in Internet Explorer is turned on the menu will not be displayed correctly. See http://windows.microsoft.com/en-us/internet-explorer/use-compatibility-view#ie=ie-10
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:
The ability to align your horizontal navigation left, center or right and to add a pixel-based margin to the sides of the nav elements has recently been added.
More actions>>Manage site>>Themes, colors and fonts>>Horizontal Navigation>>Layout.
Select Left, Center or Right.