Crème Fraîche -- Easier than you'd think!

Crème Fraîche -- it's fancy, it's gourmet, it's delicious!  But it's expensive.  Unless you are just a little patient and willing to do it yourself.

1 cup heavy cream
5 T skim milk
1 T vinegar

Mix ingredients together in glass or plastic jar.  Mix or shake well. 
Cover loosely and set aside in a warm place, stirring every 12 hours, until cream thickens (24-48 hours, typically)
When cream reaches desired thickness, place in fridge, where it will last about 7-10 days.

__________________
Yep.  That's it.  I'm not going to write a whole big thing here about why to use Crème Fraîche over sour cream or heavy whipping cream -- that's already well documented all over the internet.

I will point out that you want to find pasteurized cream, and not ultra pasteurized.  The only difference is how long it takes to age properly -- ultra pasteurized may well take >3-4 days to fully thicken.