Hunk external links plugin for WordPress lets you process outgoing links differently from internal links.
You can configure the plugin to:
- Process all outgoing links, rather than only those within your entries' content.
- Add an external link icon to outgoing links. You can use a class="noicon" attribute on links to override this.
- Open outgoing links in new windows. Note that this can damage your visitor's trust towards your site in that they can think your site used a pop-under.
- Add rel=nofollow to the links. This is not very nice for those you're linking to, but useful to avoid PR leak.
- Open outgoing links in your own page, Then you can go back.