TIgor4eg

Plugin shows <a href="http://www.twitter.com" rel="nofollow ugc">twitter</a> links from <a href="http://backtweets.com" rel="nofollow ugc">backtweets.com</a> in widget area.
<a href="http://tigor.org.ua/wp-twitbacks/" rel="nofollow ugc">Russian description</a>
It shows “shortened” links like bit.ly, goo.gl and other link shortering services.
You can configure your widget.

WP Twitter Backlinks

Plugin shows twitter links from backtweets.com in widget area.

ozhandonder

WP Link To This Post is a plugin which makes it easier for your visitors to link back to your posts. 
It creates an easy copy-paste HTML code after the post content so that a visitor can easily find the HTML code 
which can be used in other posts, pages or any part of a website.

WP Link To This Post

A simple way to make it easier for your visitors to provide backlink to your posts by giving them the HTML code they need.

WP Twitter Backlinks vs WP Link To This Post