If you just migrated your WordPress blog from one permalink structure to another, and you don’t want to lose Pagerank or traffic that accesses your blog through the old permalinks, this is for you.
Right before the old permalink generates a 404, this simple plugin grabs the slug from the end of the request and checks if there’s a post with this slug living somewhere else on your blog. If so, it will generate a “301 Moved Permanently” error, which automatically forwards your visitor (and Googlebot) to the new location of your post. It’s simple, but effective. No configuration options, just plug and play.
This plugin is in active development. All feedback is welcome on the plugins homepage.