I think plain yogurt works pretty well in chicken or tuna salad, as MM said. Other times I use low-fat mayo and just live with the fact that it doesn't taste like the regular stuff.
There's a soy product called Nayonnaise that I see at the natural food store but I haven't tried it. Can anyone here relate their experience with it?
If you're up for a challenge, it's possible to make your own mayo using canola or olive oil. It WILL have a lot of fat in it, but at least you can control the ingredients and choose a healthy fat. Also, the olive oil version will taste like olive oil, which some people find a bit weird in mayonnaise. Personally, I love it, but I'm too lazy to make it that frequently.
Here is a good basic recipe. The recipe fails to mention is that Alton recommends using pasteurized eggs for this if you can find them.