You can set all your external links are follow or nofollow by default. You can exclude some domains you want.
Auto NoFollow Links adds nofollow attribute to all external links in your blog posts automatically.
Handle External Links gives you the power to add rel="nofollow" based on header status (i.e. 404), PageRank of destination URL and many more
Allow specify rel="nofollow", "title", "class" and "id" attributes for links in visual (TinyMCE) editor.