The largest independent content sites
Quantcast makes it easy to see the largest one million sites in the US (by traffic). There's a signficant consolidation going on, with the vast majority of popular sites being owned and controlled by larger, public companies.
Because online traffic follows, as most things do, a power law curve, the top 100 sites account for a huge amount of overall web traffic–probably more than the next 900 sites combined.
After removing public companies and those that only do commerce, here are the [fewer than] thirty independent companies on the top 100: