Less sweet / lower sugar alternative to icing?
It's loaded with fat but less sweet: Cream cheese icing (example recipe) goes very well on some cakes (like a rich chocolate cake).  It does however need to be refrigerated so is best for use on cakes that are to be eaten up when they're served.  It can also be flavoured, e.g. with lemon/orange zest, with will further reduce the sweetness (and provide a nice contrast if the cake is too sweet for your tastes).

Another option is one of many variations on Cornflour buttercream (example). These use cornflour (UK)/cornstarch (US) and possibly flour, together with milk, to thicken a buttercream.  A simpler version is just to replace a little of the sugar in buttercream with cornflour, but I haven't tested whether you can significantly reduce the sweetness without affecting the texture.