The youtube formatter module provides a simple textfield display formatter for youtube links. Its especiallay lightweight, because it doesn't add a new field to the system. It's a simple displayformatter for the cores textfield.
Avalible formatters
youtube preview
This formatter will display the preview image, provided by youtube, in a selected image_style.
youtube video
Embed the video via iframe. The size and some options are avalible to configure the video.
youtube preview with colorbox integration
Displays the preview image. Onclick it will open the video inside a colorbox overlay, if colorbox is avalible.
To use the Colorbox integration, you have to install and enable the colorbox module and enable the "Colorbox load" function at the settingspage.
Instructions
Install the module on your Drupalsetup as usal. Youtube Formatter will add the three possible Formatters to the standard drupal text field! Simply add a new text field to your entity and choose the formatter you need.
The url the formatter expects is built like http://youtu.be/videoID and can be found below every youtube video, through the share button.
Similar modules
If you are looking for an own field type, you should look at: