PResponsive

This plugin will add a image below your posts, allowing your visitors to share your posts with their friends, on FaceBook, Pinterest, Google+, Twitter, Myspace, Friendfeed, Technorati, del.icio.us, Digg, Google, StumbleUpon, and other. After plugin is activated a buttons will appear after the content of the post.Clicking this button will bring the user to the bookmarking site. If the user isn’t logged in they will be prompted to do so. 
Post your articles to the social bookmarking sites and get traffic.

WP Social Bookmarking

WP Social Bookmarking plugin syndicates your blog posts in social networking sites.

andreyk

The WP Ya.Share plugin adds to your blog a set of buttons to save URLs of your pages/posts in popular social networks (twitter, facebook, livejournal, vkontakte etc.).
Добавляет блок ‘Поделиться’ (Ya.Share, разработанный в yandex) для сохранения ссылки просматриваемой страницы вашего блога в популярных ‘социальных сервисах’ (вконтакте, жж, твиттер и т. д.).
It uses the Yandex Ya.share API; I shaped it into wordpress plugin with some options only.
The following options are available:
<ul>
<li>locate “ya.share” block either in the content of a page/single post or in the sidebar (as a new widget);</li>
<li>select a list of preferable social networks;</li>
<li>button/link/icon for expanded social networks list, or nothing;</li>
<li>left, right or centered location;</li>
<li>above or under the content;</li>
<li>inscription language. 
Please feel free to contact me with your questions or suggestions.</li>
</ul>

WP Ya Share

Adds the Yandex 'Share in social networks' block into posts or widget to simplify saving URLs of your blog pages into social networks.

WP Social Bookmarking vs WP Ya Share