A simple little field type that allows the user to select a single category from a drop-down. Templating works just like the normal Categories field type. The only difference is in the entry creation page in the control panel, where you select a category from a drop-down instead of going through the pop-up element picker.
You can switch back and forth freely between this and the native Category field, as you needs change. Of course if you had multiple categories selected in the native field and you switch to Single Cat, the next time you save each entry you will lose all but the first associated category.