It looks like a Sour Orange that is used to graft Sweet Oranges on to.

Right you are, thank you! Was at my local nursery today and confirmed that it was the same plant I had.

Anigozanthos sp. Here is a link that might be useful: Kangaroo paw

My first guess was Chinotto, but I think destin is right -- the leaves aren't small enough. But they are too small (as are the fruit) for Seville. What are the petioles like? (the stem-like things that attach the leaf blade to the actual stem) -- notice on your lemon that they are cylindrical with no "wings," whereas on your grapefruit, they have big wings. (It is nice that you have these others to compare to). If there are no wings on your unknown fruit, consider rangpur lime (Citrus limonia) as a possibility. If there are substantial wings, I would still think some sort of sour orange (Citrus aurantium).