I use Hellman's Canola mayonnaise - it's lower in calories, without being 'diet.' However, the best thing I have discovered about fats is this: it's all about the mixing! When you have to add a fat and mix it, use less and mix it very, very, VERY well.
Use half what you would normally use and mix twice as long. Then taste and see if you've achieved some taste. If not, add a tiny bit more. And mix very well again - mixing is the key because you can coat the potatoes in a potato salad (for example) with a thin coating of fat, or you can make it a gloppy thick coating of fat. Your choice. The first is less caloric than the second.