This Date and Pecan Loaf is really really yummy. It doesn't have sugar in it but is sweetened with dates and bananas. I'm not sure if it would be low calorie but at least the calories that you would be eating would also include other nutrients too- like iron. I've also come across another blog with healthy sweet things. I've lost the address though so will post if I come across it.
Be careful about reducing the sugar as it affects the bake and the rise. Ground almonds contain more calories than flour and a tablespoon of oil contains more calories than a tablespoon of butter (see here)
I agree that it would be better to eat a smaller portion instead. Besides anything with less sugar/fat won't have such good keeping qualities. You could make a fatless whisked sponge, but it would have to be eaten the same day.
I would say google is your friend, a low calorie cake would just be wrong IMHO!
In terms of cutting calories I just eat a smaller piece of cake , but in terms of increasing the nutrition in a cake I do things like: substitute half the flour for wholemeal, substitute some of the flour for ground almonds, put less sugar in (most cakes taste fine with half to two thirds of the sugar listed in the recipe), use oil (rapeseed or olive) instead of butter.
You can also use fruit purees in cakes instead of some of the fat content, but it does change the texture of the cake - you'll be able to find lots of recipes online.