BibSonomy: It is run by the Knowledge Data Engineering Group of the University of Kassel, Germany. It’s specifically designed for researchers to share bookmarks and bibliographies.
BizSugar: Its similar to Reddit, but designed specifically for sharing business news and tips.
BlogBookMark: Its designed specifically for Blog hunters, it claims to have the hottest news, gossip, and blog chatter from around the web. I highly suggest that mainstream bloggers bookmark their entire sites here.
Blog Engage: Its specifically for bloggers who what to share info about blogging and tech.
Bookmarkee: Its a single dashboard to keep all of your bookmarks organized.
Bookmax: In this you can store your bookmarks and links to your favorite sites online and access them from wherever you are.
Buddymarks: Its the online personal, group and social bookmarks manager.
citeulike: This is free service to help academics to share, store, and organise the academic papers they are reading.
delicious: Delicious is the Social Bookmarking site. It allows you to easily add sites you like to your personal collection of links and to categorize those sites with keywords. Not to mention that if enough people save your site in a bookmark, it will make their popular page and send a lot of traffic. Delicious is owned by Yahoo and is a MUST for your social media and bookmarking strategy.
Design Bump: Its a site for graphic designers and photographers.
Diigo: Its a Social bookmarking on steroids.
Digg: It is social news site that changed the Internet, Digg is a high power authority and a listing in Digg for a site, even if it only has a couple of votes, will rank highly on Google and other search engines for certain terms. If your site is shared and voted upon on Digg, and makes the Digg homepage, you’ll get a lot of traffic and attention from other bloggers who read Digg.
DZone: Social bookmarking for web developers and coders.
Folkd: Folkd is a social web-service about pages, news, audios, videos, and blogs.
Flickr: Photo bookmarking site.
Google: Allows users to save and create bookmarks in their Google toolbar that can be accessed anywhere online. Google is getting more social by the day, so take advantage of their Google bookmarks and citations, because one day they probably will have some kind of influence on external meta data considered by the Google ranking algorithm.
Iloggo: A simple web-based bookmarking tool you can use for attractively displaying your favorite websites on one page.
Linkagogo: Favorites and Social Bookmarking Application, its unique dynamic tool bars automatically adapt themselves.
Markaboo: MarkaBoo is a tool for saving websites, files, and notes from your browser, email or mobile phone.
Reddit: Timely and shocking news oriented, Reddit stories are instantly voted upon and if liked by the community as a whole, can drive incredible traffic and users.
Sitebar: A solution for people who use multiple browsers or computers and want to have their bookmarks available from anywhere without need to synchronize them
Slashdot: The godfather of social news, SlashDot bookmarks are still quite powerful. Keep in mind the site has a heavy slant towards Linux and Open Source issues.
StumbleUpon: Owned by eBay, StumbleUpon is an amazing blend of social bookmarking, voting, networking, web surfing, search and blogging. Best of all, StumbleUpon can send major traffic with its userbase of around 3 million users.
Stylehive: The Stylehive is a collection of all the best products, brands, designers and stores discovered and tagged by the Hive community
Whitelinks: Securely store and quickly access favorite websites whenever connected to the Internet,
Wirefan: Social bookmarking, news articles submission site.