Some prominent sites that come to mind are Reddit, hacker news, Stack overflow, and Slashdot. Quora, although not in its present form, perhaps would fit too.
That list seems like it hasn't changed much the past few years. They all have positives and negatives. If you'd like to share your opinions of them, please do.
Niche forum sites are worth mentioning as well. They don't have to cover the range of Reddit, HN etc. They could be sites about gardening, digital photography, or science fiction. Ideally they're devoid of negativity and encourage civil discourse.
Also interested to know any websites that you visit solely for broad learning purposes. Wikipedia is the most well known example of this type of site.
