YouTube has just begun to test a user interface with new with thumbnail previews of related clips. 

It also creates a destinct url for spot in a video.  So, you don’t need to watch a whole video to see the clip of dog singing to get the good part, you can just create a location within the video.  This functionality was part of Google Video, which is shifting considerable resources to YouTube.  More enhancements to come.

— Andy Plesser