OK, ok, I understand. The title is a bit misleading. Google Reader isn’t open for 3rd party plugins, and there’s no indication that Google will ever. However, with Google Chrome extension, we can build such local “plugins”.
What are we going to achieve? Anyone uses Google Reader to read DZone feeds? I do. DZone is a very good tech news aggregator and you can vote and comment on stories. With Google Reader, you get DZone feeds like the following.