Smart Balance makes a fat free mayo that is only 10 calories a serving. I haven't tried it yet because my grocery store doesn't stock it, but it got an excellent review in Health magazine. They also recommended French's light Gourmayo and Helmann's light mayo.
I've become quite the mustard connoisseur. Right now, my fridge contains three kinds of mustard--a standard stone ground dijon that I use for cooking and Tarragon mustard and Sun-Dried Tomato Mustard for sandwiches. I look for unique flavors and varieties at specialty gourmet food stores (Cost Plus is also a good source). Since you like the horseradish spread, you might want to try some more exotic flavors of mustard.
I've never been a big mayo fan, so I don't really miss it that much on sandwiches. I'm happy with just mustard. Sometimes I also add chutney, raspberry chipotle sauce, hummus, or nonfat cream cheese.
For cooking and in salads, I use nonfat yogurt as a sub for mayo. I'm becoming somewhat of a nonfat Greek yogurt addict--I stir my cereal into it instead of using milk, eat it on Mexican food instead of sour cream, and eat it on oatmeal instead of milk. The next thing you know, I'll be spreading it on sandwiches, but I'm sure that isn't for everyone.
Mayo Substitutes?
. . . although I don't like the Fat Free variety the Low Fat isn't too bad. Totally opposite to you, I can't stand mayonnaise. 
