By default, the left ALT key drags and moves windows. How can I get rid of that?

It competes with the Alt behavior in Photoshop, so I want the left Alt key to do nothing.

You can change it from Alt to Super and vice-versa. Go to System ➜ Preferences ➜ Windows and in the Windows Preferences Choose which "Movement Key" you wish to use.

A screenshot to illustrate is placed here:

Additionally, if you are using compiz, you can change which key bindings you wish to use in order to "Initiate Window Move" when the "Enable Move Window" plugin is in use.

Here is a screenshot to illustrate what I say in the above words:

Alternatively you can press Alt+Win in Photoshop. I have tried this in Photoshop under VirtualBox machine.

You can also change the X key from command line (Terminal):

Gnome 2 (Ubuntu 11.10)

sudo apt-get install -y gconf2
gconftool-2 -t str --set /apps/metacity/general/mouse_button_modifier '<Super>'

Gnome 3 (Ubuntu 13.04)

sudo apt-get install -y libglib2.0-bin
gsettings set org.gnome.desktop.wm.preferences mouse-button-modifier '<Super>'