First, what is mascarpone?
It's a rich, creamy cow's milk cheese that is somewhere between cream cheese and butter in both texture and flavor. It's smoother than cream cheese, but thicker and less "melty" than butter; it's commonly used in Italian desserts such as tiramisu and cheesecake.
So, now in the event you have a recipe that calls for mascarpone and you can't find any available in your grocery (ahem...this has happened to me several times), here's a substitute:
Using an electric mixer, mix an 8-ounce block of softened cream cheese with 2 tablespoons heavy cream and 5 tablespoons sour cream.
Viola! Mascarpone substitute.