If you're using compiz, you can enable the "Grid" plugin to use keyboard shortcuts to move windows around the screen in a tiling sort of way.
You'll need to make sure compiz-fusion-plugins-extra is installed (sudo apt-get install compiz-fusion-plugins-extra or use synaptic - probably software center too).
I don't think either of these options have an auto tile feature though. Using a default install, I'm not sure this option is anywhere (not anywhere that I know of anyway). In that case, some of the other answers already posted might be more useful.
You could use a tiling window manager. There are many options (ion3, scrotwm, dwm, etc, etc). Search google for "tiling window manager", or search for "tiling" in synaptic.
There is also an unsupported compiz plugin called Tile.
It's in the compiz-fusion-plugins-unsupported package which can be obtained e.g. from this PPA (update of compiz from this ppa needed too)
You can watch it in action in this video (starting at 1:44)
X-tile is an application that allows you to select a number of windows and tile them in different ways.
X-tile works on any X desktop (gnome, kde, xfce, lxde…).
The main features are: many tiling geometries, undo tiling, invert tiling order, optional system tray docking and menu, filter to avoid listing some windows, filter to check some windows by default, command line interface.
If you're using compiz, you can enable the "Grid" plugin to use keyboard shortcuts to move windows around the screen in a tiling sort of way.
You'll need to make sure compiz-fusion-plugins-extra is installed (sudo apt-get install compiz-fusion-plugins-extra or use synaptic - probably software center too).
I got the tip from here: http://www.webupd8.org/2009/12/linux-w-compiz-tile-position-and-resize.html
You can do something similar without compiz using this: https://www.omgubuntu.co.uk/2010/08/python-window-organizer-organize-your-windows-using-the-keyboard
I don't think either of these options have an auto tile feature though. Using a default install, I'm not sure this option is anywhere (not anywhere that I know of anyway). In that case, some of the other answers already posted might be more useful.
You could use a tiling window manager. There are many options (ion3, scrotwm, dwm, etc, etc). Search google for "tiling window manager", or search for "tiling" in synaptic.
There is also an unsupported compiz plugin called Tile. It's in the compiz-fusion-plugins-unsupported package which can be obtained e.g. from this PPA (update of compiz from this ppa needed too)
You can watch it in action in this video (starting at 1:44)
X Tile
How-to: Set up XMonad & XMobar on Ubuntu
http://www.huntlycameron.co.uk/2010/11/how-to-set-up-xmonad-xmobar-ubuntu/
Having looked at all the answers above, the answer (without either completely dumping your existing Window Manager) seems to be "You can't".
compiz
plugins all seem to be out of dateThe only sensible option seems to be https://help.ubuntu.com/stable/ubuntu-help/shell-windows-tiled.html which isn't quite automatic but seems to be as good as it gets.
You can use Super(Win)+W with lastest Ubuntu. Press and hold Super button, you will see the hotkey tutorial
I think you can use wmtile for your case: http://ostatic.com/wmtile/home/1