Jfkeynav is a project mainly written in JavaScript, it's free.
Very basic jQuery Plugin for horizontal keyboard-navigation.
jQuery KeyNavigation Plugin
jQuery Plugin for vertical keyboard-navigation.
Setup:
init
$(document).jfkeynav({ 'position' : '#yourstartid' });
html
in order to mark up the tags you want to navigate, add the class .keynav to the corresponding tags. Additionally, each tag has to be marked up with a specific #id.