Been experimenting making a Creme Fraiche (Crème fraîche) substitute. The
best one I've come across so far is:

3 parts heavy cream (ultra pasteurized ok)
1 part *cultured* buttermilk (low fat ok)
dash lemon juice

stir thoroughly and let sit covered at room temp for at least 18 hours.
Refrigerate. Lasts at least a week.

The result is a rather light, fluffy, slightly "tart" or "sourish" creme
that's really really tasty.