There are many plugins that allow you to create custom meta boxes - the most popular of which is Custom Field Template. The basics of this should be added into core - or made a core plugin. And make it highly extensible so that it's easy to adapt.
A good example of a plugin with a good UI that does this is Simple Fields.
And no, this is not covered by Custom Taxonomies - sometimes there are things which are not part of a taxonomy which need a custom field. For example for custom field of Movies a custom field would be Review.