Thanks for your response. I guess a bit of experimentation will help determine whether they can be substitute successfully!

With regards to freezing, I meant can they be frozen as they are, ie can leftover creme fraiche be frozen, ready to use when required.

Crème fraîche is a slightly tangy, slightly nutty, thickened cream. Along with sour cream it can be used interchangeably in most recipes, but crème fraîche has two advantages over sour cream: it can be whipped like whipping cream, and it will not curdle if boiled.

Fromage frais and yogurts can be used the same way but only to compliment uncooked/cold dishes, as yogurt tends to 'break' or curdle with heat, so it has to be added at the end of cooking time.

As for freezing, except for yogurt i don't know. (We make yogurt ice cream which goes a treat with strawberrys)

Creme fraiche and fromage frais aren't readily available in Australia.  I tend always to have light sour cream in the fridge, so I use that instead of creme fraiche.  Good brands of yoghurt are easy to get, and I use it when I have it.  But sometimes I substitute the light sour cream for yoghurt (maybe with a bit of lemon juice added), because it's what I have in the house, rather than yoghurt.  If a recipe calls for fromage frais, I'd substitute low-fat cream cheese.

As Kye said, a lower-fat dairy product (like yoghurt, and especially a low-fat yoghurt) is best added right at the end of cooking, off the heat, because it will curdle when heated unless stabilized by mixing with some flour or cornflour.

What BritFinn said about cream.  In Australia we also have a lot of 'thickened cream' in the shops; it's cream with gelatine added to thicken it a bit.

Freezing dairy products like these just as they are bought doesn't really work.  They tend to split or separate, so that you sort of end up with the texture of curds and whey.  It's OK if you're going to use it in baking or something, but not very good if you want it for a sauce or a dollop on the side of food.

We can't get creme fraiche and fromage frais over here. I normally sub out the russian style sour cream for creme fraiche and a russian cream cheese for fromage frais.

I did see some small jars of creme fraiche once (along with clotted cream) and the price was the kind that invokes a sharp intake of breath and at roughly 3-4 times the price I normally pay for the russian stuff. I somehow think I will not be using those any time soon.