LinkedIn Share Button by johnbillion and Frank.Prendergast ads a LinkedIn share button to posts and pages, allowing your visitors to easily and quickly share your content with their LinkedIn network.
The plugin settings allow you to choose whether to display on:
posts on the home page
posts in archive listings
posts in search results
You can also specify whether to display inline, float left or right, and show at the top or bottom of the post. You can display the button automatically, or choose to add it on a per post basis.
Plugin homepage: http://frankprendergast.ie/resources/linkedin-button-for-wordpress/