YouTube has released a YouTube Gadget for Google Desktop. The Gadget is based on Open Source and relies on Data API feeds to search and retrieve most popular videos from YouTube for the day. The code, will help you to learn how to build your own desktop app to utilize YouTube APIs. The best part is that if have any questions or feedback, you can send an email to gd-developer AT gmail DOT com.
How about building a Advertising Gadget and interfacing with Google AdSense?
YouTube API Blog: YouTube Google Desktop Gadget