Sometimes it is useful to see if a struct hasn't been set. Of course this can get you into trouble if the unset value is a legal value, but sometimes it's not, like when dividing an int 0 will throw.
Previously I had been using code like if (divisor != new int() ) to check the value prior to calling my function.
This struck me as very odd to be newing up a struct.
Then I recalled the default keyword, which I'd only used for generics before. It turns out default works the same and declares intent better.
if (divisor != default(int) )
Of course in this contrived example it makes more sense to do != 0, but when more complex structs are involved the syntax works quite nicely.