What are some good substitutes for salt for those on low sodium diets?
There is no substitute for salt. Sorry. It has effects on the tastebuds and the body that cannot be replicated.

However! They can be approached to a degree that most people will find interchangeable. Acids can mimic the flavour-enhancing qualities of salt in a big way. The more acid you have in a dish, the less salt you will need. Beware of going too far, of course, as too much acid is just unpleasant and will need salt to counterbalance.

Heat--as in capsaicin, chili peppers--can produce much the same effect.

Echoing papin's information above; NA and K (sodium and potassium) exist in balance in the body. Too far in one direction and you have a heart attack (too much K), too much in the other and (if memory serves; I could be wrong) you essentially drown (or is it edema? I can't remember). If avoiding sodium is related to a health concern, please speak to your primary healthcare provider before adjusting your diet in this way.