404 Error Repair notifies you of any page or file requests that a user (or search engine) makes to your blog. If it can’t find the right file or page it captures that failed request into the plugin so that you can easily go and fix the issue manually.
Posts 2 Posts allows you to create many-to-many connections between posts of all kinds.
Simple Facebook Connect is a series of plugins that let you add any sort of Facebook Connect functionality you like to a WordPress blog. This lets you have an integrated site without a lot of coding, and still lets you customize it exactly the way you’d like.