I love mayonnaise - in sandwiches or burger buns, as a dip...yummy. I do, however, also love fitting into my trousers, so I try not to have too much of it too often.
A very good and tasty substitute that won't ruin the waistline is this simple little trick: mixing plain yogurt with mustard. And it couldn't be easier: for one portion, simply mix 2 heaped teaspoons of plain yogurt (2% fat or more) with a little mustard (Dijon, grainy or whatever you have) and season with salt and pepper. It's creamy and tastes just as good as mayonnaise...without the guilty conscience :-)