Is there a global menu for the XFCE panel?
Solution 1:
This instructionswork in 11.10 and earlier
No and yes. There is not one currently supported just for xfce that works (an old one exists, but is broke-see below). However, you can use the "xfce-xfapplet-plugin" xfce plugin (listed in synaptic) to put the "gnome-applet-globalmenu" or the "indicator-applet-appmenu" onto the xfce panel.
See my answer at the following link to add the "gnome-applet-globalmenu" to your setup. The instructions are for the gnome panel, but if you use the the plugin I mentioned above, you can add the "gnome-applet-globalmenu" to your xfce panel. It is a bit flaky. I found the "indicator-applet-appmenu" works better with xfce while the "gnome-applet-globalmenu" works better with the gnome-panel. Your mileage may vary.
Global - Menu applet on maverick
The ppa mentioned in the link also adds a listing in synaptic for an "xfce4-global-menu applet", but it does NOT work and according to the README is no longer supported.
Here is a screenshot of it in action (yes, I like a mac-ish look):
Solution 2:
This answer is for Ubuntu 14.04, 14.10 or 15.04 and its variants (such as Linux Mint 17.*)
There is a global menu plugin/system called TopMenu which works on Xfce and MATE. You can find precompiled packages and instructions on Web Upd8.
There are limitations, however. It will not work with Java applications, Chrom(e|ium) or LibreOffice. There are also graphical glitches with most Xfce themes. The only known compatible Xfce themes are Numix and the default Linux Mint theme.