Linked from
The 21 pages that link to Dimensionless quantity, each with the reason it gives.
RadianRelated: Arc length divided by radius cancels units, although radians remain a named SI unit.
Mass fractionRelated: Both masses use the same units, so their quotient has none.
OneRelated: Dimensionless values can equal one when compared quantities balance exactly.
Parts-per notationRelated: A parts-per value is a scaled ratio, not a unit of physical dimension.