Most of this is going to be known to long-time Ubuntu (or Gnome)
users, but if you are new to this OS or just never noticed Gnome’s
robust drag-and-drop support, this article may be of use.

Basic file operations

Let’s
start with the basics. To drag a file in the desktop or file manager,
simply hold your left mouse button while moving the mouse from the
source to the destination. Note the cursor:

• If it is an arrow, then the action will be “move
  file from source to destination”. This is the default for dragging
  files in the same file system (same partition). You can activate it by
  holding the Shift key while dragging.
• If it is a plus (+) sign, the action will be “copy
  file from source to destination”. This is the default for dragging
  files from one partition to another. You can also activate it by
  holding the Ctrl key while dragging.
• If it is a question mark (?), you will get to decide the action. This can be activated by dragging with the middle mouse button rather than the left one. In addition to “copy” and “move” you will have the option of creating a symbolic link,
  which is like a shortcut. If dragging a picture file onto the desktop,
  you will also have the option of setting it as the background.
• If
  the cursor is just the plain border, without any additional icon, no
  action will be taken. You will see the icon float back into its source
  to assure you of that. You can cancel a drag at any time by pressing
  the Esc key.

Here are some things you may not know:

• If you have a trash icon on your desktop, you can drag files there to delete them.
• You
  can have a home folder, documents, and volumes (removable media) icons
  on your desktop so that you can easily drag files into them.
• If you have a Places or Tree side panel in your file manager (it is there by default), you can drag files into those shortcuts.

Panel shortcuts

Adding
a panel shortcut is very easy if the shortcut already exists in the
menu. All you have to do is locate it in the menu, and drag it into the
panel (or the desktop, if you like having desktop shortcuts).

Icons

If
you wish to change the icon of a launcher, file or folder, you can
right click on it, and select properties. You should now see, in
addition to other information, the application’s icon in a button.
Clicking on this button will allow you to select the icon, but another
way to do it is to drag a picture (PNG is the best) right into that
button.

Web browsers

You can download files, pictures and text from browsers like Firefox using drag and drop.

If
you want to download a picture into a specific folder (or the desktop)
just drag it there. Note that this does not work with images that link
somewhere, in which case you can right click on the image, select View
Image, and then drag it.

You can copy text, or text+images as
HTML by selecting it and dragging it into a text editor, word
processor, or even another input box. What some people don’t know is
that you can drag text right into your file manager. When doing this, a
new text file will be created and you will be asked to enter its name.
I use this all the time when downloading guitar chords: just select the
text, and drag it into my chords directory.

Colors

You
can drag and drop colors in GTK applications. Wherever you see a color
selection box (a button with a colored rectangle), you can drag colors
into it or out of it. One such color selection box can be found when
you are changing your desktop background. Obviously, there are such
boxes in The Gimp, Inkscape and other graphical programs as well.
Unfortunately, Open Office is not a proper GTK application, but AbiWord
and Gnumeric are, and you can use the above method to drag various
colors in it.

To test this out on a useful example, open The
Gimp. You will see two overlapping color boxes. You can try dragging
the color from one to another to see it change. You can double click on
those boxes to see the color selection window, where you can find even
more boxes into which you can drag colors. You can see how easy it is
to drag colors from one application to another by dragging the color
from the Desktop Background window into The Gimp.

This is
particularly useful with applications like Agave. Agave is a program
that generates good-looking color schemes using complements, triads,
etc. When working with The Gimp, for example, you may wish to discover
what the complement of your current color is, and you can accomplish
this by dragging the color into Agave. You can then drag the complement
color back into one of The Gimp’s color boxes.

Another thing you
can do with colors is drag them into your file manager. This will
change the manager’s background color (beware, though; there is no
obvious way to restore the old color).

Themes

Installing
new Gnome themes is particularly easy with drag and drop. To launch the
theme manager, go to System > Preferences > Theme. Now, go to a
website that has themes (GTK, metacity or icon themes) available for
download. The easiest site to use is art.gnome.org.
After selecting the theme you wish to install, just drag and drop the
file (straight from the browser) into your theme manager window. The
theme will be installed, and you will have the option to select it.
Sites like gnome-look.org
do not give you a direct link to the file, so you will first have to
download it to your desktop, and then drag it from there to the theme
manager.

As mentioned above, to install a new wallpaper simply
drag the picture onto the desktop. Another way to do this is to right
click on the desktop, select Change Desktop Background, and then drag the picture into that window.

My Documents: Drag and Drop in Ubuntu