SixApart recently released a plugin that adds tagging to Movable Type.
The tagging is only done by the author of the post so you don’t get the benefits of social tagging like you do on delicious. However, I think it is useful for creating simple categories without having to think much about it. If the tag is already used, it gets associated with other posts. If not, voila!, a new category. Much easier interface and probably meets the needs of most bloggers who are casual about their categories.