On windows I can hold down the Windows key and press Left to make my current window take up the left half of the screen. Is there a similar app for Ubuntu?
You do not need unity for this feature which is actually implemented in the compiz grid plugin, which should also be there in older versions of Ubuntu or can be installed. Go to the comipz-config-settings-manager (
ccsm) to enable the plugin, which lets you position windows on a grid kind of corresponding your numeric keypad layout.
This is one of the features of Unity, the default graphical shell in Ubuntu 11.04, the latest version of Ubuntu. Using Ctrl Alt Numpad-4 would make your current window take up the left half of the screen.
More shortcuts are here.