Iodine

Iodine, and in particular potassium iodide tablets, have been very popular right now.

People are clearing drug stores of the stuff in anticipation of the need to ward off thyroid damage due to radioactivity from Japan.  I suspect anyone living more than 50 miles away from any of the affected plants doesn't need  to worry, but there is one thing I think deserves mention.

You don't need iodide tablets.

You can buy a bottle of tincture of iodine, 2%.  Keep it in your first aid kit for use in disinfecting wounds. 

You can also purify water with the stuff -- five to ten drops can be used to treat a liter of water.

Betadine (povidone) contains a higher concentration of iodine.  Use half the quantity you would of tincture (three to five drops per liter).

Also see here.

And if you anticipate radioactive fallout, paint a square inch of your skin with tincture of iodine. (Not sure what the absorbtion fraction of povidone is.)

And take note: If you're allergic to iodine, all bets are off.