Google announced yesterday on Google+ that Google Trends at google.com/trends now has a new feature to show you the top 200 trending videos on YouTube.
Currently it works in the US, but you can see it over here.
Google says this new report shows you what current "Internet watercooler buzzing" is taking place. The list will contain the top 200 videos that are trending on YouTube.
Might be useful for keyword ideas.
Forum discussion at Google+.