Toggling Tags is a collection of tag-related plug-ins by Josh Hughes. Rather than a one-click install, you download the collection and add the pieces you want, customizing them to match your own tags. Two are especially handy:
Toggle Tag
Use the Toggle Tag plug-in to toggle a specific tag on or off for the selected item. If that tag belongs to a group, Toggle Tag can be configured to automatically remove the others in that group.
For example, with a Priority group of Low, Medium, and High tags, you could create a Toggle Tag plug-in for each, so only one applies at a time. Assigning keyboard shortcuts to them makes this especially convenient.
Generate Toggle Tag
If you have many tags to toggle, the Generate Toggle Tag plug-in can help. It adds a submenu to the Automation menu containing all the tags you select, and you can optionally limit it to one tag at a time within a group.
For example, with a group of Weekday tags (one per day), you could generate an automation that applies just one day tag to the selected projects and actions.