Does your desktop look like the screenshot below? Are you getting tired of grabbing for the mouse or fighting with alt-tab just to switch focus to that window immediately adjacent to the one you're in?
The Directional Window Switcher (DWS) plugin for Compiz (0.9 and later only) lets you assign shortcut keys to move left, right, up, or down between windows within a viewport or (optionally) between viewports.
You have three download options:
The self-extracting bundles will install the plugin for the current user in your home directory. They are only tested as indicated above. They might work with other Ubuntu version, but I haven't tested it. If you have ABI compatibility problems with the bundles, you will need to download and install the source.
Once installed, you can use ccsm (provided by the compizconfig-settings-manager package) to enable and configure the plugin. (Beware that enabling the plugin will probably make Unity crash on account of the fact that Unity sucks.)