Add This Button and Add to Any appear to be very similar. Both services offer a small button which integrates a couple dozen services into a popup box. The small button appears to be very slick and slim, and the whole service works pretty well.
Service Links has a similar thought but displays the services as a row of buttons below the posting. This means each posting is followed by a row of little icons based on the branding of each of these services. Similarly Share-o-Matic integrates a bunch of services, they claim 63 services total. Also Share This ...
Digg.com is one of the biggest sites in this field... Digg this "This module adds a Digg this button to your nodes. If the story is not yet submitted to Digg a "Digg this story" button is displayed. If the story was already submitted a button showing the number of Diggs you received is displayed."
eKudos is a Dutch digg-like system. Scooper Ceci is a French digg-like system. Czech & Slovak Social Networks works with several Czech and Slovak systems. The hatena module integrates a Japanese social bookmarking system.
Delicious integrates with the http://del.icio.us service. It seems mostly about pulling bookmarks posted to del.icio.us into a Drupal site but has some features to assist making postings to del.icio.us.
Twitter automatically posts node summaries into the Twitter service. Drupal for Facebook automatically posts node summaries into the Facebook service.
Social Statistics shows a little banner with ranking statistics from various social bookmarking sites.
Activity Stream is an aggregator attuned to pulling data from your activity on other sites. If you have multiple blogs or post items to social bookmarking sites, Activity Stream can query those services for your activity and pull all of them into one site.
Facebook Status "This very simple module is ideal for a single-user, personal site. The fbstatus module fetches your current Facebook status, using an individualized RSS feed provided by Facebook, and provides it as a block that you can display on your site, without entanglement with the RSS aggregator."
Blog Reactions "Display Technorati, Google Blogsearch and Yahoo! Bloglines results for a node."
Technorati "enables you to use Technorati tags in your content."
If you're successful with promoting your site in these sites you may end up being 'crushed' in traffic... (the slashdot effect) .. Incoming allows you to be alerted when there is a significant change in the amount of traffic coming to your site from an outside source. ... Coral Defender "helps save your sever and bandwidth by transparently redirecting users referred from high traffic hosts to the Coral Content Distribution Network copy of your page."
Drigg is a module for Drupal to support building such a social bookmarking site, in case you want to build one of your own.