OldBlog
This is a very simple WordPress plugin that you just install and activate and it works. It looks for a ?oldblog= parameter in the URL when someone goes to your blog and, if it exists, tries to redirect you to the correct post. For example, if you go to my old Blogger blog it will redirect you do the main page here with ?oldblog=USR+OF+OLD+POST and you will be taken to the new post.