It'll put any Cocoa app in to a full screen mode - so will take away the close/zoom/hide menu bar and ALSO the system menu bar above it.... note, if you mouse over to the top of screen the app menu reappears (like an autohide option), and the same keyboard shortcut will take you back to 'standard'.
Works fine at giving you back all the vertical space on your laptop screen - but is only for Cocoa apps (install SIMBL as explained on the site)