Haidinger & Shurcliff:
Hearing about being able to see UV light after you've had a cataract operation (since apparently the eye's lens filters out UV light) made me think of other latent (or perhaps vestigial) talents in our eyes.
I remember reading in high school that we can all detect the direction of polarization of light with our unaided eye - the Haidinger's brushes phenomenon. I remember rushing out to buy polarizing filters, and sure enough, with some patience, you really could see this faint pattern. I still marvel at it when I look up at the sky, and see it faintly hanging there like a bruise. What I didn't know then is that this phenomenon not only indicates the direction of polarization, but it can also indicate the phase shift for circularly or elliptically polarized light. I haven't tested this yet...
Now the question really is whether this is a vestigial property that once conferred an evolutionary advantage to one of our ancestors, or whether this is a mere observation of a property of the dichroism of the chemicals retinal and rhodopsin in our retinas.