YouTube launched a new player for videos embedded in blogs or other sites. The player shows a list of related videos and makes it easy to move between these videos and to find the code necessary to embed a video, without opening a new page. YouTube hopes that people will spend more time watching videos.
If you find the new player too cluttered and you want to use the old one, just append &rel=0 to the URLs from the code you have to include in your site. For example, http://www.youtube.com/v/UkguiR01gMQ should be replaced with http://www.youtube.com/v/UkguiR01gMQ&rel=0.