How to remove shadow from gnome-panel?

If you want to keep shadows on windows but remove them from the panel,

Install compizconfig-settings-manager

you can use go to Window decoration

and set Shadow windows to (any) & !(type=Dock).

You can do this with compizconfig-settings-manager

sudo apt-get install compizconfig-settings-manager

run it and go window decoration settings. Set window shadow to 'none'.

NOTE: This solution is for the Unity panel in Ubuntu 11.04+, while the question was asked for gnome-panel.

Replace /usr/share/unity/3/panel-shadow.png with an invisible image. In other words, edit that PNG file and make it 100% transparent and remove the black shade in the image. The panel shadow will then disappear after a logout/login.

You could also create a new PNG file with a width of 1 px (height could be anything, preferably 20px) in gimp , and remove the background white layer. The entire PNG will then be transparent, and then you could put this PNG in /usr/share/unity/3/panel-shadow.png.