When you writing a post, and if you are using tags, it can be tiresome to choose relevant terms from the post content. To help with that, GD Content Tools Pro plugin includes Tagger addon that will help with terms extraction.
For every non-hierarchical taxonomy, the plugin adds expansion of the meta box on the post edit page with options to suggest terms based on your content. To do that, the plugin can use one of the online term extraction services or simple term extraction script.
Plugin supports these online services:
For each one, you need API key (all are with free registration, but with paid plans also). They can give you relevant or contextual terms based on the content.