WILO is a front-end plugin that allows you to find internal linking opportunities for your content. This is not an automatic plugin; it's designed for SEOs who don't want the process automated.
I use this daily on WordPress sites that I look after to improve my internal linking. After trying some of the automated internal link plugins, I was not too fond of the fact that they were *too* automated. This plugin might take a little longer to use, but it results in better SEO.
WordPress Internal Link Optimiser