OK, I'll go first.
I swear by SMBC, I love to bake from scratch. But sometimes, I just don't have the ambition to make a filling.
If I need a fast caramel filling, I use pre-made Dulce de Leche in a can, made by Nestle. It is soft, and creamy, and pipes easily into cupcakes, spreads wonderfully on cake layers.
For quick curd fillings, I use Robertson's Curds in jars. It's imported, and the best pre-made that I have found in supermarkets. I am sure there might be other brands available in specialty shops, but this is the one for me. I only use curd with sponge cakes, which can take the extra moisture.
Robertson's also beats very nicely into SMBC. I use a half jar to a batch of SMBC. It adds flavor without changing the texture of the icing, and is not overwhelming in taste.
I have scattered shelled cocoa nibs over an icing filling, to give the middle a bit of crunch. I have also used crushed chocolate-covered espresso beans this way. Jimmies also work, but tend to soften after a while.
If I want to add fruit flavors to SMBC for a filling, I either use icing fruit or Smucker's Simply Fruit.
Nutella is good, but I only use a thin layer of it between cake layers, on top of SMBC, because of it's consistency. It pipes nicely into cupcakes.
When I do make filling, it's almost always fruit or ganache. I buy crushed pineapple, drain the juice, then cook the juice to about half. See Merissa's raspberry filling recipe for more details:
Then I add the cornstarch with water slurry, and cook it a bit more. When it is thick, then I add the pineapple. This way the pineapple keeps some of it's crunch.
I use her recipe with fresh raspberries, strawberries, blackberries, blueberries, and black cherries. Works every time. Thanks, Merissa!
Otherwise, it's plain SMBC or ganache for filling.