Reddit is the obvious one, especially as if you sign up you can get rid of the sub-Reddits that don't interest you or are too noisy (e.g. atheism, politics, world news, videos) and subscribe to only those that suit your tastes.
If you want something a bit more high-brow and substantial out of the gate, however, MetaFilter is a classic example: http://www.metafilter.com/