Post Indexer is super powerful.
Wp Admin Tag/Categories - You could have a box that uses jquery/ajax to autocomplete a tag or category that has occured on another blog.
Global Tag/Cat Sitemap - a display of posts by a specified tag/cat on the WPMUDEV network.
Related Posts - Show related posts at the end of the post that are not only from the same blog but others. Could be tied into a quick Global Search, with the initial keywords being extracted from the current post they are viewing.
If anyone thinks any of this is cool please +1. If not, feel free to imagine and post your thoughts here.