Tags are great, they help you search and identify your inventory however you wish. However, without proper management they can get out of control. Sometimes you want to get a bird's eye view of everything in your account.
While there is no "Tag Export", you can easily see all tags in use by exporting an inventory report, such as Inventory Export - Stock. This report has a column with all of the tags assigned to your inventory. 
After exporting you can use your filters to isolate only the items with tags, extract the tags column, and use the "remove duplicates" function in Excel to get a concise list of the tags currently in use in your inventory system.