I created a plugin that automatically adds user-specified tags to WordPress posts. It is very useful if you are too lazy to add tags yourself and I read somewhere that tagging posts is also helpful for SEO (perfect for autoblogs).
Download from WP Plugin Directory: Automatic Post Tagger
In comparsion with other plugins this one supports adding tags by searching for one or more strings. It does not use any external API - it just adds tags that you specify in its settings.