It depends on the platform you are designing the control for. If you are doing it in a touch interface the best mechanism for creating may not be the same for an online application or a desktop application. That being said…
I follow the “keep it simple, stupid” approach for stuff like this. When adding tags, use a free-form text area and let people separate tags with a comma. After tags are added, display them below the add control/field as separate objects on the page with an X next to each one so they can easily be deleted. Don’t allow editing – if they mistyped a tag, then the user can delete the typo and re-add the tag.