The sugar free dessert offerings vary from restaurant to restaurant. A lot of them will bring you a sugar free brownie or cheesecake that they've obviously had in the freezer. Le Cellier actually has quite a lovely citrus mousse that's sugar free, but it's not the menu. Ask for it when you order your dinner, because it takes a while to make.
Another favorite of ours is the sugar free chocolate mousse at Norway's Kringla Bakery og Kafe - really good!