WoorkUp has an interesting post on oEmbed, and how you can use jQuery to take something like a YouTube or Flickr URL and automatically load the video or image on the page. Facebook uses this technique to fetch thumbnails and descriptions when you post a link.
WordPress 2.9 also includes oEmbed functionality, allowing you to easily add YouTube videos to your posts, simply by wrapping the video page’s URL in a pair of “embed” shortcode tags.
WoorkUp shows how you can implement the feature in your own projects.
Learning oEmbed: Convert Links Into Embedded Content [WoorkUp]