Touchpad Indicator is a small applet developed by Lorenzo Carbonell, which allows you to turn easily on your laptop touchpad. Especially if it bothers you when you type and you already have a mouse connected.
To install on Ubuntu the touchpad Indicator (supported only at this time by Ubuntu 10.10 Maverick Meerkat), open a terminal and run the following command:
sudo add-apt-repository ppa:lorenzo-carbonell/atareao
sudo apt-get update
sudo apt-get install touchpad-indicator
Once installed, go to the Applications menu -> Accessories then Touchpad Indicator. It is also possible to define a shortcut key to toggle the feature even faster.
Launchpad page available here.