It is sometimes that I use Ubuntu in home as primary OS, a shortcut that I am used to use is ALT+Space to minimize, maximize or restore a window. But in Ubuntu or maybe in Gnome, It is my first time that i use Gnome as primary desktop, this key combination act differently.
When I pressed ALT+Space as i used to do it and it showd a semi transparent window which let me select one of application links that are present in Gnome/Ubuntu menu a link to some web site, a folder in my home directory or something else which i have not descovered yet.
The opened window allows you shows all menu items that begins with your typed character and let you select your target shortcut
Here is an screenshot of this window. a wonderful thingy, by the way maybe it is not Gnome built in feature and one of additional packages that i installed add it to Gnome.