From Cook's Illustrated, Crème Fraîche is used a lot in recipes but not always easy to find. Here's their recipe, interestingly they say it should be "pourable" but the stuff I buy in grocery stores has the consistency of cream cheese.
Ingredients
1 cup heavy cream (avoid ultra pasteurized)
2 tbsp buttermilk
Instructions
Stir together ingredients and place in warm location. 75 to 80 degrees is ideal, lower temperatures will lengthen fermentation time. Leave until thickened but still pourable, 12 to 24 hours.