You can get "no added sugar" chocolates, and some of them taste really good (the Cavalier ones for example) but they all tend to give you the runs....
In fact, really good quality dark chocolate, the 70% cocoa solids stuff, hasn't got an awful lot of sugar in anyway, so I treat myself to one square of that occasionally (at the moment Lindt dark chocolate with Chilli - mmmm!). Portion control is the problem!
Incidentally, if you like chocolate mousse, they had some recipes in the Guardian today, a couple of which had no sugar in (apart from what is in the chocolate already). I might give those a go myself.
As for cakes, I made a chocolate cake using Da Vinci sugar free syrup and ground almonds as flour for my birthday and it worked really well. I also made Baklavas with the Da Vinci syrup and although they weren't quite as sticky as the real thing, they were really popular! So, don't give up hope, experiment with the sweeteners!