You can download YouTube for Windows Phone directly on your device from the Windows Phone Marketplace or by using the link which follows:
Price | Free |
Requirements | Windows Phone 7 or above |
The application which actually connects you to the mobile version of the YouTube provides HTML5 streaming on your Windows Phone devices. With the HQ playback quality you can enjoy the HD videos from YouTube the way they were meant to be played.. The YouTube app for Windows Phone also integrates with the Music and Videos hub so you can access your most recently watched videos from the hub itself.
source -WPCentral