The story goes that Hamilton figured out the definition of quaternions while walking across Broom Bridge in Dublin.
What I didn't know is that there's a plaque there commemorating this. The text of the plaque says:
"Here as he walked by on the 16th of October 1843, Sir William Rowan Hamilton in a flash of genius discovered the fundamental formula for quaternion multiplication i2 = j2 = k2 = ijk = -1" carved (?) on a stone of this bridge."
There's also a sign commemorating ENIAC, the "first computer", across the street from my office. I didn't know it was there until about a year after I came to Penn, because it was obscured by construction. It says "ENIAC, the Electronic Numerical Integrator and Computer, was invented by J. Presper Eckert and John Mauchly. It was built here at the University of Pennsylvania in 1946. The invention of this first all-purpose digital computer signaled the birth of the Information Age." Mark Jason Dominus pointed me to a picture in the Wikipedia article.
What other signs do you know of that say, roughly, "math happened here"?