Operator overloading -- lets collect some use cases

[Don]

There's been some interesting discussion about operator overloading over 
the past six months, but to take the next step, I think we need to 
ground it in reality. What are the use cases?

I think that D's existing opCmp() takes care of the plethora of trivial 
cases where <, >= etc are overloaded. It's the cases where the 
arithmetic and logical operations are overloaded that are particularly 
interesting to me.

The following mathematical cases immediately spring to mind:
* complex numbers
* quaternions (interesting since * is anti-commutative, a*b = -b*a)
* vectors
* matrices
* tensors
* bigint operations (including bigint, bigfloat,...)
I think that all of those are easily defensible.

But I know of very few reasonable non-mathematical uses.
In C++, I've seen them used for iostreams, regexps, and some stuff that 
is quite frankly bizarre.

So, please post any use cases which you consider convincing.