Whats a good light or fat free substitution for sour cream?
I'm not sure exactly what you're asking - whether you want brands of sour cream whose reduced-fat version is as tasty as the full-fat version, or if you want to substitute something else in place of sour cream.

For the latter, it depends somewhat on what you're using it for. I find that in a lot of dips, substituting plain lowfat or fat-free yogurt works fine. It might result in a thinner consistency than if you use sour cream, but it generally works.

If you're looking for a substitute that has the consistency of sour cream, try lowfat or fat-free Greek yogurt. It's thicker than regular yogurt and much closer to sour cream's consistency. I sometimes do this is with dips or when baking (Greek yogurt works very well as a substitute for sour cream in sour cream coffee cakes, for example). 

Substitutes typically work well when the sour cream flavor isn't all that important anyway. In this case, you could use the light or fat-free versions of sour cream and probably not notice much of a flavor difference.

If you're looking to replace sour cream as a condiment, however (on tacos, burritos, or as a topping for soup), you'll definitely notice a different taste if you use regular or Greek yogurt. For many things, a yogurt substitution still works; it just tastes different.

As far as brands of lowfat or fat-free sour cream that taste as good as the full-fat versions, that's an extremely subjective question and would probably be limited based on your locations. As such, it's not a very good fit for this site.