I just stumbled on this site while refreshing myself on the use of Slater's rules for calculating Zeff for electrons. There are a variety of calculators on there including some for visualizing lattice planes and diffraction, equilibrium, pH and pKa, equation balancing, Born-Landé, radioactive decay, wavelengths, electronegativities, Curie Law, solution preparation crystal field stabilization energy, and more.
I checked and it calculated Zeff correctly but I can't vouch for the accuracy of any of the other calculators.
This is not a good teaching website but would be good for double checking math
I used this to double check my Slater's rules calculations (and found a mistake in my answer key!)