Powerful quake hits off Indonesia's Sumatra island

On Digg, users share intersting online content by submitting links to the site. At that point, the Digg audience can vote on whether or not they think it is interesting. Articles with lots of votes, or "diggs," rise up higher on the site's main page and topical subsection pages. Another form of social sharing, this site also lets users categorize the content they are submitting to Digg and label it with descriptions of up to 350 characters. Digg users can also submit comments on each content item submitted to the site.

To register, go to: http://digg.com/register

At its most basic level, del.icio.us allows users to save their bookmarks online. Del.icio.us also gives users the ability to "tag" their bookmarks with descriptive category names. For example, someone who has bookmarked multiple Web pages that deal with the Washington Nationals baseball team could tag those links with any terms they want, like "baseball," "nationals," "natsfan," etc.

As members of a "social bookmarking" community, del.icio.us sers can also see how many other people have bookmarked the same pages, and they can look at those users' bookmark collections to find other interesting online content.

To register, go to: http://del.icio.us/register

Reddit allows users to submit news articles and other online content to the site. Users also give articles a thumbs-up or thumbs-down. Reddit then uses those votes to build a user profile and to find articles to recommend to you. Users can also submit comments on items posted to the site.

To register, go to: http://reddit.com/login