There are several online sites that can be data-mined to reveal really nice trends, top-10s and topdown summaries. Twitter is the archetype site for this, thanks to hashtags making an easy job for anyone wanting to investigate trends. I prefer Reddit for datamining specific trends as it powered by folk having arguments on topics they are passionate about as opposed to ideas of celebrities, corporate spokespeople and ФСБ agents.
eBay is also fun as it reveals what people are willing to pay for things. But the best source of data, even for other datasets, is Wikipedia. Not only to read up on things, but also to get data for things within a given "category".
