From the Official Google Blog, check out What Were We Watching This Year? Let's Rewind 2011. Here you'll see a YouTube video of what was the most popular. Scroll down to see a list of music videos, particular channels and categories, and topics such as fashion, how-to, and sports. Google reports that there were over 1 trillion playbacks on YouTube. That's an incredibly high number, more than twice as many stars in the Milky Way, says Google.
If you're more into still images, check out Mashable's article on Flickr 2011: The Year in Photos. Political unrest was a popular subject as was weather-related events. Occupy Wall Street makes an appearance as does Harry Potter. The collection is also available directly on Flickr with brief commentary by Yahoo editors.