A plaster wall in the old Kamerlingh Onnes Laboratory at Leiden University, covered with the signatures of some of the greatest physicists of the 20th century. Paul Ehrenfest, the Austrian-Dutch physicist who held Leiden's chair of theoretical physics, started the tradition of asking visiting scientists to sign the wall of his office after giving a lecture.
The result is a who's who of modern physics: Albert Einstein, Niels Bohr, Enrico Fermi, Paul Dirac, Werner Heisenberg, Wolfgang Pauli, and dozens more. The signatures span from the 1920s to the present day, and the tradition continues — recent Nobel laureates have added their names alongside the giants. The wall is inside the university's physics building and access depends on whether the building is open, but it is sometimes included in university tours. Worth asking at the reception desk.