I know grid/grid2 can do it (with lua editing of course).
Doing this in GridFrame.lua:
Code:
Line 129: self:SetBackdropBorderColor(0, 0, 0, 1)
Line: 132: self:SetBackdropColor(0, 0, 0, .3)
and then in the in-game settings, changing background color to 0 should give you basic transparency.