TinyNav.js is a tiny jQuery plugin (443 bytes minified and gzipped) that converts <ul> and <ol> navigations to a select dropdowns for small screen. It also automatically selects the current page and adds selected="selected" for that item.

This isn't the first plugin to do this and it doesn't provide a lot of options, but it might be the smallest (file size). Scale this page to see the plugin in actio