There are plenty of butter substitutes on the market, but usually these "diet" products have chemical or artificial ingredients in them. They use the words "natural" and "healthy" on the label, but if you read the ingredients you will find otherwise.
The most natural alternative to butter is olive oil...a very healthy, beneficial fat, high in vitamin C and antioxidants.
If you don't want to switch to olive oil and completely give up the taste of butter, here's a compromise my mother taught me years ago. Combine equal parts real butter (NOT margarine or imitation spread) and olive oil.
It's really very easy, and if you use a light tasting olive oil, you won't even notice the difference in flavor. (If you prefer more olive flavor, use a stronger oil.)
Place two sticks of butter in a bowl and leave at room temperature until softened. Add 1 cup light extra virgin olive oil. Mix with electric mixer until well blended. Pour into a 2 cup container with lid and refrigerate until hard.
The calorie count is about the same.
Olive oil is a little bit higher in total fat (14g compared to 11g in butter) but that's because it is high in mono unsaturated fat, which is a good fat.
Olive oil has only 2g of saturated fat compared to 7g in butter.
Another important benefit: olive oil has no sodium. Compare this to the 90mg of sodium in butter.
The taste of butter with added health benefits and half the guilt. Enjoy!