Hi, I second this request. Specifically:
Android mobile app: need a tag picker. Currently, users have to type in an exact tag. If you misspell, a duplicate tag gets created. I usually forget exactly what I called a tag (e.g. did I call it "Design & Development" or "Development & Design"?)
Windows app: need a tag picker. Currently, there is an auto-suggest as you type, but it would be easier to just select from a list of existing tags (and ability to type a few characters to narrow that list)