Go to Google Trends, plot some searches and then on the left hand side click on the "limit to" option and select "YouTube."
You can then see the search trends on YouTube search.
For example, here is the spike in interest on YouTube for [harlem shake]:
And while cat videos are more popular than dog videos on YouTube, recently goat videos have been the talk of the town:
Google keyword ideas for SEOs and publishers come from Google Trends.
Forum discussion at Google+.