Social bookmarking, basically linking to your site from a bunch of stumbleupon accounts and other bookmarking sites (reddit, etc.)
bookmarking is a very key tool for me in SEO because it allows users to help promote and also bookmark there favorite sites online
Not only can you save your favorite websites and send them to your friends, but you can also look at what other people have found interesting enough to tag. Most social bookmarking sites allow you to browse through the items based on most popular, recently added, or belonging to a certain category like shopping, technology, politics, blogging, news, sports, etc. You can even search through what people have bookmarked by typing in what you are looking for in the search tool. In fact, social bookmarking sites are being used as intelligent search engines.