Hocus Pocus Buttons is a WordPress-Plugin that provides sidebar widget with a widerange set of buttons for social bookmarking websites. However each button is only visible to the user if he has visited the corresponding site lately.
The goal is not to disturb the user by using too many buttons / links (since he could probably only use one or two at all) and rather only show relevant services.
For information about the use of this plugin visit the plugin page