I never us anything but butter, whether I am on a diet or not. There is simply no alternative.
But like others I find there are many cases where something entirely different can be used. I find if I am having anything on toast, not just scrambled eggs, but baked beans, tomatoes etc. The toast does not need to be buttered. In sandwiches, quark makes a good substitute for butter, or even a little bit of pesto. It is high in flavour, so very little is needed. Quark and a spoonful of parmesan makes a good baked potato filler.
I think the answer is keep using butter, but where possible use something entirely different.