Galileo is reputed to have demonstrated that unequal masses fall at an equal rate by dropping them off the tower of Pisa and observing that they hit the ground simultaneously. Has anyone tried to push this experiment to the limit at small scale or is the result observable from some other experiment?
The principle of universality of free fall (or weak equivalence principle, WEP) states that all bodies fall with the same acceleration, independent of mass and composition.
In page 12 of this link there is a list of measured limits using two different isotopes: rubidium isotopes give the best precision, 10^-15.
In this link an ambitious experiment is described which will try to measure whether antihydrogen ( antimatter) follows WEP or has a different g than hydrogen (matter) even though they have the same mass. In this link there is a simple description of the experiment.
So yes , people are still experimenting and measuring even at the level of atoms trying to improve the precision of the equivalence.