This week I signed up for reddit. My Google Reader had accumulated 5 or 6 subreddits, so I was pretty much using the site already. The same thing happened with Twitter - I was following 6 or 7 people through Reader and finally decided it was time to give back.
The site is basically a much better, more filtered version of Digg. It's not as good-looking, but it's way more functional. You subscribe to different topics you are interested in and the main page aggregates all these "subreddits" on the main page, so the articles that show up should at least be relevant. You are able to vote articles up or down and comment. You can also submit new articles, or submit a general question for fellow redditors to use. There are 5 tabs on the top of each reddit: what's hot, new, controversial (voted equally up and down), top (best of), saved (your bookmarks in that reddit). These can lead to really cool hive mind things, like a list of best TED talks.
My reddit subscriptions are mostly for tech stuff: reddit.com, politics, technology, programming (proggit), science, linux, cogsci, Python, javascript, Ubuntu, hardware, compsci, cyberlaws, tedtalks, java, PHP. If you join up, I'm JaimieMurdock.
Because reddit is not responsible for lost productivity, I've set a 20 minute limit for every 6 hours in LeechBlock, which is in effect all day every day. It takes some enforced self-control not to be consumed ;)