The NZ Plot
The N-Z plot, also known as the Plot of the Nuclides, is a standard plot in which one shows $N$, the number of nucleons in an isotope, against $Z$, its proton number. For low $A=N+Z$, this is approximately a straight line along which stable nuclei exist.
As an medical physicist, I perpetually find myself wishing to refer to the NZ plot occasionally, in order to find out if a given nuclide is either (a) NMR visible (that is, $I\neq0$), (b) radioactive with a decent activitiy, or occasionally both. Undoubtedly the best online interactive viewer is JANIS – a Java-based Nuclear Data Information System provide by the OECD’s Nuclear Energy Agency. You can download it here.
Perhaps because I live somewhat out of data, I like enjoying actually seeing the relatively low $Z$ part of the N-Z plot on a wall next to my desk. I’ve therefore produced a freely-distributible printable version that is just about readable if printed out as two A3 sheets and then stuck together. You can find this in both PDF form (warning: large, browser-breakingly good) and as a probably more accesible rendered PNG ($9441\times6712$). It’s far from a wonderful work of art, but perhaps it would be useful to someone else too.
The Semi-Empirical Mass Formula (SEMF)
If you’re interested in a bit of third-year undergraduate physics, quite a lot of the N-Z plot is “explained” by the Semi-Epirical Mass Formula, or SEMF.