TinyNav.js is a tiny jQuery plugin (443 bytes minified and gzipped) that converts <ul> and <ol> navigations to a select boxes for small screen. It also automatically selects the current page and adds selected="selected" for that item. There's also a Wordpress plugin available, here.
This isn't the first plugin to do this and it doesn't provides a lot of options, but it might be the smallest (file size).
Check out also the new version called Responsive Nav.
For instructions and demo go to http://tinynav.viljamis.com
Licensed under the MIT license.
Copyright (c) 2011-2012 Viljami Salminen, http://viljamis.com/
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
v1.1 (2012-11-14) - Adds an optional "label" tag before the "select" tag, thanks to @larcher
v1.05 (2012-10-19) - Adds the support for multiple navigation depths, thanks to @nicoandrade
v1.04 (2012-10-19) - Adds the possibility to change the text of the header. Notice that there seems to be a bug in jQuery 1.8.0 which prevents this script from working correctly if there is no selected item in the navigation (the plugin does work with older and newer versions correctly though, currently tested up to 1.8.2).
v1.03 (2012-03-15) - Adds new option called "header", thanks to @jorde
v1.02 (2012-03-07) - Some performance improvements and a bug fix
v1.01 (2012-01-04) - Code Refactoring
v1.00 (2011-12-31) - Release
Great! New ideas are more than welcome, but please check the Pull Request Guidelines first before doing so.