Does Android put wifi and other data networks to sleep when the display is turned off? I was wondering if stock Android puts Wifi and other data networks to sleep when the display of the device is turned off. If not, what app can help me do the same ?
It depends, some Android devices have an advanced setting WiFi Sleep Policy under

Settings -&gt; Wireless -&gt; Wi-Fi Settings -&gt; [Menu Button] Advanced -&gt; Wi-Fi Sleep Policy

But I am not sure if this is a vanilla Android feature or an manufacturer modification. At least the Galaxy S series has this setting.

Data networks are usually fully active. But Apps like Tasker allow you to define certain profiles to improve battery life. For example: Only allow 2G when the display is off, because 2G requires less battery then 3G (UMTS).