What is a good substitute for creme fraiche?

Depending on the dish (and where you live) you can also use Mexican "Crema"; south of the border sour cream. I have no trouble finding it in my local supermarkets.

Make your own if you have the time:

1 cup heavy cream
2 Tablespoons sour cream

In a small mixing bowl, thoroughly combine the heavy cream and sour cream. Cover the bowl with plastic wrap and let stand at room temperature for 12 to 24 hours.

Or Vt. Butter and Cheese fat free fromage blanc.

Depends on what you're using it for, but if it's something like a garnish for soup, I'll use whole milk Greek yogurt.

Sour cream works well as a substitute