Stick it ! brings a useful feature to every WordPress theme that's usually present only in few premium themes.
Give the plugin the selector (class/id) of the navigation menu you want to stick and it's width and it will stick it to the top of browser window making it easy for your readers to navigate your site.
Name: Stick it !
Product demo page: http://Demos.SwiftThemes.Com
Product info page: http://askwebexpert.com/stick-it/