DiggDigg as a news site that allows anyone to submit to the system. Users then vote, or 'digg' up articles. Popular articles gain thousands of unique visits. Although the main crowd is IT professionals, it's slowly growing it's target group
Alexa Ranking: 
Rank 160
Estimated Traffic: 33433760 Visits
Google Pagerank: Pagerank: 8
Yahoo! BuzzYahoo Buzz is Yahoos attempt at bringing the Digg style website to the masses. Although not as popular as the aforementioned Digg it’s slightly easier to gain a high ranking site.
Alexa Ranking: Rank 2Estimated Traffic: 124469244 VisitsGoogle Pagerank: Pagerank: -1
StumbleUponStumbleUpon allows users to use a browser toolbar to 'stumble' across new websites. These websites are then voted up or down. Each user slowly builds a portfolio which the system uses to work out what sites to send users to.
Alexa Ranking: Rank 435Estimated Traffic: 4418609 VisitsGoogle Pagerank: Pagerank: -1
DeliciousDelicious allows people to take there bookmarks with them anywhere. Bookmarks are also shared, with the more people that bookmark a site via Delicious, the higher than site ranks.
Alexa Ranking: Rank 659Estimated Traffic: 1623083 VisitsGoogle Pagerank: Pagerank: 8
RedditReddit is a source for what's new and popular online. Each link is voted upon and then as a result is ranked. Like Digg the results on the homepage are the most popular and thus most visited.
Alexa Ranking: Rank 757Estimated Traffic: 0 VisitsGoogle Pagerank: Pagerank: 8
MixxMixx allows for users to submit and vote upon stories. Unlike Digg mixx also generates custom reporting for each user, meaning the results are tailored to each person. It's easy to submit to Mixx.
Alexa Ranking: Rank 464Estimated Traffic: 879108 VisitsGoogle Pagerank: Pagerank: -1
Propeller.comPropeller.com was previously known as Netscape.com. It is a is a social news aggregator operated by AOL-Netscape. It is similar to Digg; users can vote for which stories are to be included on the front page and may comment on them as well.
Alexa Ranking: Rank 2477Estimated Traffic: 1164549 VisitsGoogle Pagerank: Pagerank: 8
SlashdotSlashdot.org is a much more technology focused social bookmarking service. Like Digg it counts on user votes, however not all stories are posted stright to the site. Great for technology related news!
Alexa Ranking: Rank 1721Estimated Traffic: 862416 VisitsGoogle Pagerank: Pagerank: -1
DMOZ.orgDMOZ is an open directory edited by thousands of people around the world. It's very hard to get into the DMOZ project, but the rewards when it comes to SEO are worth it.
Alexa Ranking: Rank 789Estimated Traffic: 1218438 VisitsGoogle Pagerank: Pagerank: -1
TechnoratiBlog search, breaking news, photos, and videos, on the latest headlines, business, entertainment, lifestyle, politics, sports, and technology. See what and who is most popular in the blogosphere.
Alexa Ranking: Rank 620Estimated Traffic: 3309174 VisitsGoogle Pagerank: Pagerank: 9
KaboodleKaboodle is a social shopping community where people discover, recommend and share products. Kaboodle's powerful shopping tools allow people to organize their shopping through lists, discover new things from people with similar style, get discounts on popular products and find best prices.
Alexa Ranking: Rank 1433Estimated Traffic: 3941212 VisitsGoogle Pagerank: Pagerank: 6
Fark.comFark is a news aggregator and an edited social networking news site. Every day Fark receives 2,000 or so news submissions from its readership.
Alexa Ranking: Rank 3612Estimated Traffic: 652762 VisitsGoogle Pagerank: Pagerank: 4
NewsvineNewsvine is updated continuously by citizens like you, Newsvine is an instant reflection of what the world is talking about at any given moment.
Alexa Ranking: Rank 5660Estimated Traffic: 1142779 VisitsGoogle Pagerank: Pagerank: -1
Clipmarks.comOn Clipmarks.com, you can see clips of text, images or video about all sorts of topics that other people find while surfing the web. The idea is that through each other, we can learn more, know more and enjoy more than we could possibly do alone.
Alexa Ranking: Rank 7050Estimated Traffic: 379354 VisitsGoogle Pagerank: Pagerank: 6
Diigo (used to be Furl)Diigo is a powerful research tool and a knowledge-sharing community. Diigo is two services in one -- it is a research and collaborative research tool on the one hand, and a knowledge-sharing community and social content site on the other.
Alexa Ranking: Rank 4440Estimated Traffic: 146144 VisitsGoogle Pagerank: Pagerank: 6
Faves.comFaves.com’s free website delivers recommendations personalized to each user’s topics and people of interest. Through a unique Topic Network, Faves.com users are connected to other members who make the highest quality recommendations in their respective areas of interest.
Alexa Ranking: Rank 6665Estimated Traffic: 159051 VisitsGoogle Pagerank: Pagerank: 6
DzoneDZone is a free link-sharing community for developers. Anyone can submit new links to the incoming queue. Members vote on upcoming links to determine what gets promoted. Everyone can browse, search and comment on links.
Alexa Ranking: Rank 5483Estimated Traffic: 281098 VisitsGoogle Pagerank: Pagerank: 6
BlinklistBlinkList is a powerful productivity tool that makes is much easier for anyone to share and save their links for later. With BlinkList you can save a local copy of any web page on your computer. That way, even if the web page disappears, you will always have a local copy that you can get back to later.
Alexa Ranking: Rank 6152Estimated Traffic: 86650 VisitsGoogle Pagerank: Pagerank: 7
SimpySimpy is a social bookmarking service. With Simpy, you can save, tag and search your own bookmarks and notes or browse and search other users' links and tags. You can be open and share your links with others, or keep them private.
Alexa Ranking: Rank 14210Estimated Traffic: 54581 VisitsGoogle Pagerank: Pagerank: 6
Alexa Ranking: Rank 33366Estimated Traffic: 40967 VisitsGoogle Pagerank: Pagerank: 4