Category Order, does just that.

Ever needed a quick and easy way to change the order in which categories are displayed on your blog or website? I had a tough case involving a custom query displaying posts from a specific category, then broken down into sub-categories. Up to there, all was well, but then I found the need to change the order of appearance of those sub-categories. Category Order hasn’t been updated since 2008, but it still works like a charm. I’ve even activated it network-wide on a multisite install with no problem. In a word: love. Thanks wp_guy, whoever you are!