Flickr, Viddler, Qik, Pownce and Revision3 are the first services to support oEmbed, an easy way to allow embeding media from a certain URL in a third party site.

From the oEmbed site:

oEmbed is a format for allowing an embedded representation of a URL on third party sites. The simple API allows a website to display embedded content (such as photos or videos) when a user posts a link to that resource, without having to parse the resource directly.

This means that if you for example find a nice photo on flickr, you can take the URL and easily turn it into embed-able data:

Original URL: http://flickr.com/photos/codepo8/2475016321/

oEmbed URL: http://flickr.com/services/oembed?url=http://flickr.com/photos/codepo8/2475016321/

Result:

XML:
  1. <oembed>
  2.   <version>1.0</version>
  3.   <type>photo</type>
  4.   <title>? too much myspace error</title>
  5.   <author_name>codepo8</author_name>
  6.   <author_url>http://www.flickr.com/photos/codepo8/</author_url>
  7.   <cache_age>3600</cache_age>
  8.   <provider_name>Flickr</provider_name>
  9.   <provider_url>http://www.flickr.com/</provider_url>
  10.   <width>500</width>
  11.   <height>375</height>
  12.   <url>
  13.     http://farm4.static.flickr.com/3128/2475016321_982666ec95.jpg
  14.   </url>
  15. </oembed>

You can define the output format and the maximum width and height with URL parameters:

oEmbed URL:

http://flickr.com/services/oembed?url=http://flickr.com/photos/codepo8/2475016321/&format=json&maxwidth=200

Result:

JAVASCRIPT:
  1. {
  2.   version: '1.0',
  3.   type: 'photo',
  4.   title: '? too much myspace error',
  5.   author_name: 'codepo8',
  6.   author_url: 'http://www.flickr.com/photos/codepo8/',
  7.   cache_age: '3600',
  8.   provider_name: 'Flickr',
  9.   provider_url: 'http://www.flickr.com/',
  10.   width: '100',
  11.   height: '75',
  12.   url: 'http://farm4.static.flickr.com/3128/2475016321_982666ec95_t.jpg'
  13. }

Supported formats for responses so far are photo, video, link and rich.