Tran Ngoc Tuan Anh

This plugin adds rel=”nofollow” to all external links in post content. So, your flow of PageRank can not run to other websites and your blog’s SEO is improved.
The internal links are unchanged.
Requires PHP5.

External Nofollow

cubecolour

A simple and lightweight plugin to indicate external http and https links in posts, pages and custom content by appending a small diagonal arrow icon to the link.
No configuration is required just install and activate.

Indicate External Links

Very reliable has survived many Wp updates and just keeps working.

This plugin just keeps on working

Indicates external links in post, page and custom content.

External Nofollow vs Indicate External Links