Oxygen Appmenu is an Oxygen style for Kwin that displays the application menu as a button in the titlebar and works with Qt and GTK applications as well as Firefox or Thunderbird:
The menu comes with different styles: 'menu' (first screenshot in the post), an icon or the application name:
Install Oxygen Appmenu
Important: according to its KDE-Look page, Oxygen Appmenu "works, but has some 'no compositing' issues".
Update: the PPA below now requires KDE 4.8, to install it, see: KDE 4.8 Released, Install It In Kubuntu 11.10
Oxygen Appmenu needs Appmenu-Qt and Qt patched to work with Appmenu-Qt. These are already available in the Ubuntu repositories, so to install it in Ubuntu 11.10 Oneiric Ocelot, you can use a PPA (the same PPA used in the Icon Tasks post):
sudo add-apt-repository ppa:gnumdk/ppa sudo apt-get update sudo apt-get dist-upgrade sudo apt-get install kwin-style-oxygen-appmenu appmenu-qt
Arch Linux users can install Oxygen Appmenu via AUR.
For other Linux distributions, get the Oxygen Appmenu source via KDE-Look.
Once installed, open System Settings and under Workspace Appearance > Window Decorations, select "Oxygen Appmenu", then log out and log back in.
Here, you can also choose between different menu styles, configure its background color and so on, by clicking "Configure decoration":
Thanks to Georgi Karavasilev for the tip!