Horizontal CSS Dropdown Menus
http://www.cssnewbie.com/horizontal-dropdown-menus/

Glazebrook, Rob L.
CSSnewbie
2008

Abstract:

Last week, CSSnewbie reader Andrea Pluhar wrote in with an interesting problem: she wanted to use CSS dropdown menus like the ones we featured last week on a website that she was building, but the design called for the submenu to be arranged horizontally, not vertically. She sent me a mockup of what she was after (excerpted above) and wondered if there was a way to accomplish this effect using CSS.